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#### Question 1:

If the sides of a triangle are 3 cm, 4 cm and 6 cm long, determine whether the triangle is a right-angled triangle.

#### Answer:

We have,

In order to prove that the triangle is a right angled triangle we have to prove that square of the larger side is equal to the sum of the squares of the other two sides.

Here, the larger side is .

Hence, we have to prove that.

Let solve the left hand side of the above equation.

Now we will solve the right hand side of the equation,

Here we can observe that left hand side is not equal to the right hand side.

Therefore, the given triangle is not a right angled triangle.

#### Question 2:

The sides of certain triangles are given below. Determine which of them are right triangles.

(i) a = 7 cm, b = 24 cm and c = 25 cm
(ii) a = 9 cm, b = 16 cm and c = 18 cm
(iii) a = 1.6 cm, b = 3.8 cm and c = 4 cm
(iv) a = 8 cm, b = 10 cm and c = 6 cm

#### Answer:

(i) Let

In order to prove that the given sides of a certain triangle forms a right angled triangle we have to prove that square of the larger side is equal to the sum of the squares of the other two sides.

Here, the larger side is.

Hence, we have to prove that.

Let solve the left hand side of the above equation.

Now we will solve the right hand side of the equation,

Here we can observe that left hand side is equal to the right hand side that is .

Therefore, the given sides of a certain triangle form a right angled triangle.

(ii) Let

In order to prove that the given sides of a certain triangle forms a right angled triangle we have to prove that square of the larger side is equal to the sum of the squares of the other two sides.

Here, the larger side is .

Hence, we have to prove that.

Let solve the left hand side of the above equation.

Now we will solve the right hand side of the equation,

Here we can observe that left hand side is not equal to the right hand side.

Therefore, the given sides of a certain triangle do not form a right angled triangle.

(iii) Let

In order to prove that the given sides of a certain triangle forms a right angled triangle we have to prove that square of the larger side is equal to the sum of the squares of the other two sides.

Here, the larger side is .

Hence, we have to prove that.

Let solve the left hand side of the above equation.

Now we will solve the right hand side of the equation,

Here we can observe that left hand side is not equal to the right hand side.

Therefore, the given sides of a certain triangle do not form a right angled triangle.

(iv) Let

In order to prove that the given sides of a certain triangle forms a right angled triangle we have to prove that square of the larger side is equal to the sum of the squares of the other two sides.

Here, the larger side is .

Hence, we have to prove that .

Let solve the left hand side of the above equation.

Now we will solve the right hand side of the equation,

Here we can observe that left hand side is equal to the right hand side that is .

Therefore, the given sides of a certain triangle form a right angled triangle.

#### Question 3:

A man goes 15 meters due west and then 8 meters due north. How far is he from the starting point?

#### Answer:

Let us draw the diagram. Let A be the starting point. From point B he goes to the north.

Therefore, we obtained the following drawing.

­­­Now we have to find how far is he from the starting point that is we have to find .

Now we will use Pythagoras theorem to find the length of AC.

………(1)

Let us substituting the values of AB and BC in equation (1) we get,

Let us take the square root we get,

Since AC is the distance therefore it should be positive.

Therefore, he is from the starting point.

#### Question 4:

A ladder 17 m long reaches a window of a building 15 m above the ground. Find the distance of the foot of the ladder from the building.

#### Answer:

Let us draw the diagram from the given information we get a right angled triangle ABC as shown below,

Let the window be at the point A. We know that angle formed between the building and ground is always 90°.

Given: AB = 15 m and CA = 17 m

Now we will use Pythagoras theorem to find .

Let us substitute the values we get,

Subtracting 225 from both the sides of the equation we get,

Let us take the square root we get,

Therefore, the distance of the foot of the ladder from the building is.

#### Question 5:

Two poles of heights 6 m and 11 m stand on a plane ground. If the distance between their feet is 12 m, find the distance between their tops.

#### Answer:

Let us draw the diagram from the given information.

Let us draw a perpendicular from B on CD which meets CD at P.

It is clear that BP = 12 m because it is given that distance between feet of the two poles is 12 m.

After drawing the perpendicular we get a rectangle BACP such that AB = PC and BP = AC.

Because of this construction we also obtained a right angled triangle BPD.

Now we will use Pythagoras theorem,

Let us substitute the values of BP and PD we get,

Taking the square root we get,

Therefore, distance between the top of the two poles is .

#### Question 6:

In an isosceles triangle ABC, AB = AC = 25 cm, BC = 14 cm, Calculate the altitude from A on BC.

#### Answer:

We know that altitude that is a perpendicular drawn on the unequal side of the isosceles triangle bisects that side.

Therefore, BD = DC = 7 cm.

Let us use the Pythagoras theorem in right angled triangle ADB we get,

Substituting the values we get,

Subtracting 49 from both the sides we get,

Let us take the square root we get,

AD = 24 cm

Therefore, the altitude of the isosceles triangle is .

#### Question 7:

The foot of a ladder is 6 m away from a wall and its top reaches a window 8 m above the ground. If the ladder is shifted in such a way that its foot is 8 m away from the wall, to what height does its tip reach?

#### Answer:

The given information can be represented as follows.

Here, A is the position of the window and AC is the ladder.

Also, DE is the same ladder when it is shifted.

C and E are the original and final position of the foot of the ladder.

Now, applying Pythagoras theorem in ΔABC,

AC2 = AB2 + BC2

⇒ AC2 = (8 m)2 + (6 m)2 = (10 m)2

⇒ AC = 10 m

Now, again applying Pythagoras theorem in ΔEBD

DE2 = EB2 + BD2

⇒ (10 m)2 = (8 m)2 + BD2

⇒ BD2 = 100 m2 − 64 m2 = 36 m2

⇒ BD = 6 m

Thus, the tip of the ladder is now at the height of 6 m above the ground.

#### Question 8:

Two poles of height 9 m and 14 m stand on a plane ground. If the distance between their feet is 12 m, find the distance between their tops.

#### Answer:

Let us draw the diagram from the given information.

As we are given that distance between their feet is 12 m

.

Now we get a right angled triangle DCE.

Let us applying the Pythagoras theorem we get,

Substituting the values we get,

Let us take the square root we get,

Therefore, distance between their top is .

#### Question 9:

Using Pythagoras theorem determine the length of AD in terms of b and c shown in the given figure.

#### Answer:

In ∆ABC and ∆DBA,

(90º each)

(Common)

Therefore, by AA-criterion for similarity, we have .

Now we will substitute the values of AC and AB

We are finding the value of AD therefore; we will use the following ratios,

Now we will multiple both sides of the equation by .

We will simplify the above equation as below,

..…(1)

But we know that substituting the value of BC in equation (1) we get,

Therefore, the value of AD in terms of b and c is.

#### Question 10:

A triangle has sides 5 cm, 12 cm and 13 cm. Find the length to one decimal place, of the perpendicular from the opposite vertex to the side whose length is 13 cm.

#### Answer:

Since we obtained two right angled triangles, and .

In and

(Common angle)

So, by AA-criterion

Now we will multiply both sides of the equation by.

.....(1)

Let us simplify the equation (1) as given below,

Now we will substitute the values of BC, AB and AC.

Therefore, the length of the altitude is.

#### Question 11:

ABCD is a square. F is the mid-point of AB. BE is one third of BC. If the area of ∆FBE = 108 cm2, find the length of AC.

#### Answer:

It is given that F is the midpoint of AB. Therefore, we have AF = FB.

It is also given that               .....(1)

Now look at the figure. Quadrilateral ABCD is a square and hence all angles are of 90º.

In , and hence it is a right angle triangle.

We know that the area of the right angle triangle is $\frac{1}{2}$ × base  × height

Therefore, $Ar\left(∆FBE\right)=\frac{1}{2}×BF×BE$

cm2

Now we will multiple both sides of the equation by 2 we get,                   .....(2)

But we know that and.

Let us substitute these values in equation (2) we get,

Let us simplify the above equation as below,

But we know that ABCD is a square and hence AB = BC = CD = AD.

.....(3)

We know that 216 is the cube of 6 therefore we can write the equation (3) as below,

AB2 = 63 × 6

AB2 = 64

Therefore, side of the square ABCD is 36 cm.

Now we are going to find the diagonal AC.

Diagonal of the square can be calculate by using the formula given below,

Diagonal = $\sqrt{2}$ Side

.....(4)

We know that

Let us substitute the value of in equation (3).

Therefore, the length of AC is .

#### Question 12:

In an isosceles triangle ABC, if AB = AC = 13 cm and the altitude from A on BC is 5 cm, find BC.

#### Answer:

We have given an isosceles triangle and we know that the altitude drawn on the unequal side of the isosceles triangle bisects that side.

Therefore, in and

$\angle B=\angle C$      (Equal sides have equal angles opposite to them)

(AAS congruence theorem)

Now we will use Pythagoras theorem in right angled triangle ADB.

Let us substitute the values of AB and AD we get,

Subtracting 25 from both sides we get,

Since

Therefore, length of BC is.

#### Question 13:

In a ∆ABC, AB = BC = CA = 2a and AD ⊥ BC. Prove that

(i) $\mathrm{AD}=a\sqrt{3}$
(ii)

#### Answer:

In and

$\angle B=\angle C$        (60º each)

$\angle ADB=\angle ADC$           (90º each)

(AAS congruence theorem)

But therefore, we get,

………(1)

Now we will divide both sides of the equation (1) by 2, we get,

Now we will use Pythagoras theorem in right angled triangle ADB.

Now we will substitute the values of AB and BD we get,

Therefore, .

We have given an equilateral triangle and we know that the area of the equilateral triangle is.

Here, side is 2a

Therefore, .

#### Question 14:

The lengths of the diagonals of a rhombus are 24 cm and 10 cm. Find each side of the rhombus.

#### Answer:

We know that a quadrilateral is said to a rhombus if all sides of the quadrilateral are equal. Diagonals of a rhombus bisect each other at right angles.

Quadrilateral ABCD is a rhombus and diagonals AC and BD intersect at point O.

As we defined above, we get , , and angle .

We are given that AC = 10 cm and BD = 24 cm.

Therefore, we get, AO = OC = 5 cm and BO = OD = 12 cm.

Now we will use Pythagoras theorem in the right angled triangle AOD as below,

…......(1)

Now we will substitute the values of AO and OD in equation (1) we get,

Let us take the square root

AD = 13

Therefore, length of the side of the rhombus is .

#### Question 15:

Each side of a rhombus is 10 cm. If one its diagonals is 16 cm find the length of the other diagonal.

#### Answer:

We know that a quadrilateral is said to a rhombus if all sides of the quadrilateral are equal. Diagonals of a rhombus bisect each other at right angles.

Quadrilateral ABCD is a rhombus and diagonals AC and BD intersect at point O.

As we defined above, we get , , and angle .

We are given that AB = 10 cm and AC = 16 cm. Now we will find length of BD.

As we know the definition of rhombus we get AB = BC = CD = AD.

Therefore, we get, AB = BC = CD = AD = 10 cm

Also we know that diagonals of rhombus bisect each other at right angles therefore, we get,

and

Here, we know the length of AC therefore, we get, .

Now we will use Pythagoras theorem in the right angled triangle AOD as below,

…......(1)

Now we will substitute the values of AD and AO in equation (1) we get,

…......(2)

Now we will subtract 64 from both sides of the equation (2)

Now we will take the square root.

OD = 6

We know that

Therefore, length of the other diagonal is .

#### Question 16:

In an acute-angled triangle, express a median in terms of its sides.

#### Answer:

Let ΔABC be acute angled triangle where AD is its median with respect side BC.

It is known that in any triangle, the sum of the squares of any two sides is equal to twice the square of half of the third side together with twice the square of the median which bisects the third side.

This is the required expression.

#### Question 17:

Calculate the height of an equilateral triangle each of whose sides measures 12 cm.

#### Answer:

We are asked to find the height of the equilateral triangle.

Let us draw the figure. Let us draw the altitude AD. We know that altitude is also median of the equilateral triangle.

Therefore,.

In right angled triangle ADB, we will Pythagoras theorem, as shown below,

Now we will substitute the values.

144 = AD+ 36

AD2 = 144 − 36 = 108

Taking square root, we get

AD = 10.39 cm

Therefore, the height of the equilateral triangle is.

#### Question 18:

In right-angled triangle ABC is which ∠C = 90°, if D is the mid-point of BC, prove that AB2 = 4 AD2 − 3AC2.

#### Answer:

∆ABC is a right-angled triangle with ∠C = 90°. D is the mid-point of BC.

We need to prove that.

Join AD.

Since D is the midpoint of the side BC, we get

BD = DC

Using Pythagoras theorem in triangles right angled triangle ABC

.....(1)

Again using Pythagoras theorem in the right angled triangle ADC

.....(2)

From (1) and (2), we get

Hence, .

#### Question 19:

In the given figure, D is the mid-point of side BC and AE ⊥ BC. If BC = a, AC = b, AB = c, ED = x, AD = p and AE = h, prove that :

(i) ${b}^{2}={p}^{2}+ax+\frac{{a}^{2}}{4}$
(ii) ${c}^{2}={p}^{2}-ax+\frac{{a}^{2}}{4}$
(iii) ${b}^{2}+{c}^{2}=2{p}^{2}+\frac{{a}^{2}}{2}$

#### Answer:

(i) It is given that D is the midpoint of BC and.

Therefore, …......(1)

Using Pythagoras theorem in the right angled triangle AED,

…......(2)

Let us substitute , and in equation (2), we get

Let us take another right angled triangle that is triangle AEC.

Using Pythagoras theorem,

…......(3)

Let us substitute and in equation (3) we get,

Here we know that and .

Substituting , and we get

…......(4)

From equation (1) we can substitute in equation (4),

…......(5)

(ii) Using Pythagoras theorem in right angled triangle AEB,

…...... (6)

We know that AB = c and AE = h now we will find BE.

Therefore,

We know that and substituting these values in we get,

Now we will substitute AB = c, AE = h and in equation (6) we get,

…......(7)

Let us rewrite the equation (7) as below,

…......(8)

From equation (1) we can substitute in equation (8),

…......(9)

(iii) Now we will add equations (5) and (9) as shown below,

Therefore, .

#### Question 20:

In the given figure, ∠B < 90° and segment AD ⊥ BC, show that

(i) ${b}^{2}={h}^{2}+{a}^{2}+{x}^{2}-2ax$
(ii)

#### Answer:

(i) Since AD perpendicular to BC we obtained two right angled triangles, triangle ADB and triangle ADC.

We will use Pythagoras theorem in the right angled triangle ADC

…......(1)

Let us substitute AD = h, AC = b and DC = (a − x) in equation (1) we get,

…......(2)

(ii) Let us use Pythagoras theorem in the right angled triangle ADB as shown below,

…......(3)

Let us substitute AB = c, AD = h and BD = x in equation (3) we get,

Let us rewrite the equation (2) as below,

…......(4)

Now we will substitute in equation (4) we get,

Therefore, .

#### Question 21:

In ∆ABC, ∠A is obtuse, PB ⊥ AC and QC ⊥ AB. Prove that:

(i) AB ✕ AQ = AC ✕ AP
(ii) BC2 = (AC ✕ CP + AB ✕ BQ)

#### Answer:

Given: ΔABC where BAC is obtuse. PB AC and QCAB.

To prove: (i) AB × AQ = AC × AP and (ii) BC2 = AC × CP + AB × BQ

Proof: In ΔACQ and ΔABP,

∠CAQ = BAP (Vertically opposite angles)

∠Q = P (= 90°)

∴ ΔACQ ΔABP [AA similarity test]

In right ΔBCQ,

⇒ BC2 = CQ2 + QB2

⇒ BC2 = CQ2 + (QA + AB)2

⇒ BC2 = CQ2 + QA2 + AB2 + 2QA × AB

⇒ BC2 = AC2 + AB2 + QA × AB + QA × AB                  [In right ΔACQ, CQ2 + QA2 = AC2]

⇒ BC2 = AC2 + AB2 + QA × AB + AC × AP                  (Using (1))

⇒ BC2 = AC (AC + AP) + AB (AB + QA)

⇒ BC2 = AC × CP + AB × BQ

#### Question 22:

In a right ∆ABC right-angled at C, if D is the mid-point of BC, prove that BC2 = 4(AD2 − AC2).

#### Answer:

It is given that ∆ABC is a right-angled at C and D is the mid-point of BC.

In the right angled triangle ADC, we will use Pythagoras theorem,

…......(1)

Since D is the midpoint of BC, we have

Substituting in equation (1), we get

#### Question 23:

In a quadrilateral ABCD, ∠B = 90°, AD2 = AB2 + BC2 + CD2, prove that ∠ACD = 90°.

#### Answer:

In order to prove angle it is enough to prove that .

Given,

.....(1)

Since, so applying Pythagoras theorem in the right angled triangle ABC, we get

.....(2)

From (1) and (2), we get

Therefore, angle .              (Converse of pythagoras theorem)

#### Question 24:

In an equilateral ∆ABC, AD ⊥ BC prove that AD2 = 3BD2.

#### Answer:

We have to prove that .

In right angled, using Pythagoras theorem we get,

….....(1)

We know that in an equilateral triangle every altitude is also median.

Therefore, AD bisects BC.

Therefore, we have

Since is an equilateral triangle,

Therefore, we can write equation (1) as

…......(2)

But

Therefore, equation (2) becomes,

Simplifying the equation we get,

Therefore, .

#### Question 25:

∆ABD is a right triangle right-angled at A and AC ⊥ BD. Show that

(i) AB2 = BC . BD
(ii) AC2 = BC . DC
(iii) AD2 = BD . CD
(iv) $\frac{{\mathrm{AB}}^{2}}{{\mathrm{AC}}^{2}}=\frac{\mathrm{BD}}{\mathrm{DC}}$

#### Answer:

(i)

In and ,

$\angle ACB=\angle A=90°$

(Common angle)

So, by AA criterion

.....(1)

(ii) In and ,

(Common angle)

So, by AA criterion

.....(2)

(iii) We have shown that is similar to and is similar to therefore, by the property of transitivity is similar to .

.....(3)

(iv) Now to obtained AB2/AC2 = BD/DC, we will divide equation (1) by equation (2) as shown below,

Canceling BC we get,

Therefore,

#### Question 26:

A guy wire attached to a vertical pole of height 18 m is 24 m long has a stake attached to the other end. How far from the base of pole should the stake be driven so that the wire will be taut?

#### Answer:

We will draw the figure from the given information as below,

Let AB be the vertical pole of length 18 m and let the stake be at the point C so the wire will be taut.

Therefore, we have , and we have to find BC.

Now we will use Pythagoras theorem,

Let us substitute the values we get,

Subtracting 324 from both sides of the equation we get,

We can rewrite the 252 as , therefore, our equation becomes,

Now we will take the square root,

Therefore, the stake should be far from the base of the pole so that the wire will be taut.

#### Question 27:

An aeroplane leaves an airport and flies due north at a speed of 1000 km/hr. At the same time, another aeroplane leaves the same airport and flies due west at a speed of 1200 km/hr. How far apart will be the two planes after ?

#### Answer:

Let us draw the figure first.

An aeroplane which flies due north at a speed of 1000 km/hr covers the distance AB after hr and another aeorplane that flies due west at the speed of 1200 km/hr covers the distance BC after hr.

We know that

Let us calculate AB first as shown below,

Similarly we can calculate BC.

Now we have find AC. To find AC we will use Pythagoras theorem,

Taking square root we get,

Therefore, after hrs the aeroplanes will be approximately far apart.

#### Question 28:

Determine whether the triangle having sides (a − 1) cm, and (a + 1) cm is a right angled triangle.

#### Answer:

Let

Larger side is

We know that any number plus 1 is always greater than that number minus 1 and product of 2 and its square root.

For example : If a = 36

If a = 5

In order to prove that the given sides forms a right angled triangle we have to prove that .

Let us solve the left hand side first.

Now we will simplify the right hand side as shown below,

We can see that left hand side is equal to right hand side.

Therefore, the given sides determined the right angled triangle.

#### Question 1:

In each of the figures [(i)-(iv)] given below, a line segment is drawn parallel to one side of the triangle and the lengths of certain line-segment are marked. Find the value of x in each of the following :

(i)

(ii)

(iii)

(iv)

#### Answer:

In each of the figure, we have to find the value of x

(i)

By cross multiplication on both sides, we get

Hence the value of is .

(ii)

By cross multiplication on both sides, we get

Hence the value of is .

(iii)

By cross multiplication on both sides, we get

Hence the value of is .

(iv)

By cross multiplication on both sides, we get

Hence the value of is .

#### Question 2:

What values of x will make DE || AB in the given figure?

#### Answer:

If , then

Hence, the value of is .

#### Question 3:

In ∆ABC, points P and Q are on CA and CB, respectively such that CA = 16 cm, CP = 10 cm, CB = 30 cm and CQ = 25 cm. Is PQ || AB?

#### Answer:

Given: and, we get

We will check whether or not to conclude whether.

$\frac{CP}{AC}=\frac{10\mathrm{cm}}{16\mathrm{cm}}=\frac{5}{8}\phantom{\rule{0ex}{0ex}}\frac{CQ}{CB}=\frac{25\mathrm{cm}}{30\mathrm{cm}}=\frac{5}{6}\phantom{\rule{0ex}{0ex}}\therefore \frac{CP}{AC}\ne \frac{CQ}{CB}\phantom{\rule{0ex}{0ex}}$

Hence, PQ is not parallel to AB.

#### Question 4:

In the given figure, DE || BD. Determine AC and AE.

#### Answer:

Given, .

In ∆ABC and ∆ADE

$\angle ADE=\angle C$
(Corresponding angles)

$\angle A=\angle A$     (Common)

(AA Similarity)

Hence the value of and is

#### Question 5:

In the given figure, given that ∆ABC ∼ ∆PQR and quad ABCD ∼ quad PQRS. Determine the value of x, y, z in each case.

#### Answer:

(i) We have,

So the ratio of the sides of the triangles will be proportional to each other.

Therefore put the values of the known terms in the above equation to get,

On solving these simultaneous equations, we get

(ii) We have,

So the ratio of the sides of the quadrilaterals will be proportional to each other.

Therefore put the values of the known terms in the above equation to get,

On solving these simultaneous equations, we get

#### Question 6:

In ∆ABC, P and Q are points on sides AB and AC respectively such that PQ || BC. If AP = 4 cm, PB = 6 cm and PQ = 3 cm, determine BC.

#### Answer:

In triangle ABC, P and Q are points on sides AB and AC respectively such that.

In ΔAPQ and ΔABC,

So, $∆\mathrm{APQ}~∆\mathrm{ABC}$      (AA Similarity)

$\frac{AP}{AB}=\frac{PQ}{BC}$

Substituting value and , we get

By cross multiplication we get

Hence, the value of BC is .

#### Question 7:

In each of the following figures, you find who triangles. Indicate whether the triangles are similar. Give reasons in support of your answer.

(i)

(ii)

(iii)

(iv)

(v)

#### Answer:

(i) In two triangles, we observe that

In similar triangle corresponding sides are proportional to each other.

Therefore, by SSS-criterion of similarity,

two triangles are similar

(ii)

PQ || BC             (Corresponding angles formed are equal)

In ΔAPQ and ΔABC,

So, $∆\mathrm{APQ}~∆\mathrm{ABC}$      (AA Similarity)

two triangles are similar

(iii) In two triangle, we observe that

In ΔABC and ΔCDE

$\frac{CD}{CE}=\frac{CB}{CA}$

$\angle ACB=\angle DCE$    (Vertically opposite angles)

ΔABC $~$ ΔCDE        (SAS Similarity)

two triangles are similar

(iv) In two triangle, we observe that

In two triangles corresponding sides are not proportional to each other.

two triangles are not similar.

(v) In two triangle, we observe that

In two triangles corresponding sides are not proportional to each other.

two triangles are not similar.

#### Question 8:

In ∆PQR, M and N are points on sides PQ and PR respectively such that PM = 15 cm and NR = 8 cm. If PQ = 25 cm and PR = 20 cm state whether MN || QR.

#### Answer:

Given and .

So, by the converse of basic proportionality theorem MN || QR.

#### Question 9:

In ∆ABC, P and Q are points on sides AB and AC respectively such that PQ || BC. If AP = 3 cm, PB = 5 cm and AC = 8 cm, find AQ.

#### Answer:

In, P and Q are points on sides AB and AC respectively such that

Then we have

and

Hence the value of AQ is

#### Question 10:

In the given figure, ∆AMB ∼ ∆CMD; determine MD in terms of x, y and z.

#### Answer:

We are given ∆AMB ∼ ∆CMD

We have to determine the value of MD in terms of x, y and z.

Given

$⇒\frac{BM}{MD}=\frac{AM}{CM}\phantom{\rule{0ex}{0ex}}\frac{x}{MD}=\frac{y}{z}$

By cross multiplication we get

Hence, the value of MD is .

#### Question 11:

In ∆ABC, the bisector of ∠A intersects BC in D. If AB = 18 cm, AC = 15 cm and BC = 22 cm, find BD.

#### Answer:

We have to find the value of BD.

Given and .

In , AD the bisector of .

On cross multiplication, we get

BD = 12 cm

Hence, the value of BD is .

#### Question 12:

In the given figure, l || m

(i) Name three pairs of similar triangles with proper correspondence; write similarities.
(ii) Prove that $\frac{\mathrm{AB}}{\mathrm{PQ}}=\frac{\mathrm{AC}}{\mathrm{PR}}=\frac{\mathrm{BC}}{\mathrm{RQ}}$

#### Answer:

(i) Three pair of similar triangles are-

$∆ABK~∆PQK$       (AAA Similarity)

$∆CBK~∆RQK$       (AAA Similarity)

$∆ACK~∆PRK$        (AAA Similarity)

(ii) Since the pair of similar triangles mentioned above can give us the desired result. The ratios of the corresponding side of the similar triangle are equal.

So,

Therefore,

……equation (1)

Similarly in ,

……equation (2)

Similarly ,

……equation (3)

From the above equations 1 and 2 we have,

=

……eqaution (4)

Combining it with equation (4)

hence proved

#### Question 13:

In the given figure,

AB || DC prove that

(i) ∆DMU ∼ ∆BMV
(ii) DM × BV = BM ✕ DU

#### Answer:

(i) Given

In triangle DMU and BMV, we have

Each angle is equal to 90°

Each are vertically opposite angles.

Therefore, by AA-criterion of similarity

(ii)Since

By cross multiplication, we get

Hence proved that

#### Question 14:

ABCD is a trapezium in which AB || DC. P and Q are points on sides AD and BC such that PQ || AB. If PD = 18, BQ = 35 and QC = 15, find AD.

#### Answer:

In trapezium ABCD, AB || DC. P and Q are points on sides AD and BC such that PQ || AB.

Join AC. Suppose AC intersects PQ in O.

From (1) and (2), we get

Hence, the value of AD is .

#### Question 15:

In ∆ABC, D and E are points on sides AB and AC respectively such that AD ✕ EC = AE ✕ DB. Prove that DE || BC.

#### Answer:

Given: In, D and E are points on sides AB and AC such that

To Prove:

Proof:

Since

∴ DE || BC             (Converse of basic proportionality theorem)

#### Question 16:

ABCD is a trapezium having AB || DC. Prove that O, the point of intersection of diagonals, divides the two diagonals in the same ratio. Also prove that $\frac{\mathrm{ar}\left(∆\mathrm{OCD}\right)}{\mathrm{ar}\left(∆\mathrm{OAB}\right)}=\frac{1}{9}$, if AB = 3 CD.

#### Answer:

We are given ABCD is a trapezium with AB||DC

Consider the triangles AOB and COD in which

Therefore,

Hence we have proved that O, the point of intersection of diagonals, divides the two diagonals in the same ratio.

We are given AB = 3CD and we have to prove that

We already have proved that AOB and COD are similar triangles

So

Hence, Prove that

#### Question 17:

Corresponding sides of two triangles are in the ratio 2 : 3. If the area of the smaller triangle is 48 cm2, determine the area of the larger triangle.

#### Answer:

The ratio of the areas of two similar triangles is equal to the ratio of the square of any two corresponding sides.

.

Hence the area of the larger triangle is .

#### Question 18:

The area of two similar triangles are 36 cm2 and 100 cm2. If the length of a side of the smaller triangle in 3 cm, find the length of the corresponding side of the larger triangle.

#### Answer:

Since the ratio of areas of two similar triangles is equal to the ratio of the squares of any two corresponding sides.

(Corresponding side of larger triangle)2 =

(Corresponding side of larger triangle)2 =

(Corresponding side of larger triangle)2 = 25

⇒ Corresponding side of larger triangle = 5

Hence, the length of the corresponding side of the larger triangle is

#### Question 19:

Corresponding sides of two similar triangles are in the ratio 1 : 3. If the area of the smaller triangle in 40 cm2, find the area of the larger triangle.

#### Answer:

Since the ratio of areas of two similar triangles is equal to the ratio of the squares of any two corresponding sides.

Hence the area of the larger triangle is

#### Question 20:

In ∆ABC, AD and BE are altitude. Prove that $\frac{\mathrm{ar}\left(∆\mathrm{DEC}\right)}{\mathrm{ar}\left(∆\mathrm{ABC}\right)}=\frac{{\mathrm{DC}}^{2}}{{\mathrm{AC}}^{2}}$.

#### Answer:

Given: ΔABC in which AD and BE are altitudes on sides BC and AC respectively.

Since ADB = AEB = 90°, there must be a circle passing through point D and E having AB as diameter.

We also know that, angle in a semi-circle is a right angle.

Now, join DE.

So, ABDE is a cyclic quadrilateral with AB being the diameter of the circle.

∠A + BDE = 180° [Opposite angles in a cyclic quadrilateral are supplementary]

⇒ ∠A + (BDA + ADE) = 180°

⇒ ∠BDA + ADE = 180° − A ..... (1)

Again,

∠BDA + ADC = 180° [Linear pair]

⇒ ∠BDA + ADE + EDC = 180°

⇒ ∠BDA + ADE = 180° − EDC ..... (2)

Equating (1) and (2), we get

180° − A = 180° − EDC

⇒ ∠A = EDC

Similarly, B = CED

Now, in ΔABC and ΔDEC, we have

∠A = EDC

∠B = CED

∠C = C

∴ ΔABC ΔDEC

#### Question 21:

The diagonals of quadrilateral ABCD intersect at O. Prove that $\frac{\mathrm{ar}\left(∆\mathrm{ACB}\right)}{\mathrm{ar}\left(∆\mathrm{ACD}\right)}=\frac{\mathrm{BO}}{\mathrm{DO}}$.

#### Answer:

We are given the following quadrilateral with O as the intersection point of diagonals

To Prove:

Given ACB and ACD are two triangles on the same base AC

Consider h as the distance between two parallel sides

Now we see that the height of these two triangles ACB and ACD are same and are equal to h

So

Now consider the triangles AOB and COD in which

Therefore,

From equation (1) and (2) we get

Hence prove that

#### Question 22:

In the given figure, each of PA, QB, RC and SD is perpendicular to l. If AB = 6 cm, BC = 9 cm, CD = 12 cm and PS = 36 cm, then determine PQ, QR and RS.

#### Answer:

Given and

PA, QB, RC and SD is perpendicular to l,

Therefore, by the corollory of basic proportionality theorem, we have

Now for QR

Again for RS

Hence, the values of PQ, QR and RS are respectively.

#### Question 23:

In ∆ABC, ray AD bisects ∠A and intersects BC in D. If BC = a, AC = b and AC = c, prove that

(i) $\mathrm{BD}=\frac{ac}{b+c}$
(ii) $\mathrm{DC}=\frac{ab}{b+c}$

#### Answer:

Given: In ray AD bisects angle A and intersects BC in D, If and

To Prove:

(i)

(ii)

(i) The corresponding figure is as follows

Proof: In triangle ABC, AD is the bisector of

Therefore

Substitute and we get,

By cross multiplication we get.

We proved that

(ii) Since BC = CD + BD

#### Question 24:

In each of the figures given below, an altitude is drawn to the hypotenuse by a right-angled triangle. The length of different line-segment are marked in each figure. Determine x, y, z in each case.

(i)

(ii)

#### Answer:

(i) is right angled triangle right angled at B

is right triangle right angled at D

is right triangle right angled at D

By canceling equation and by elimination method, we get

y canceling and by elimination method we get

Now, substituting in equation (iv) we get

Now, substituting in equation (ii) we get

Hence the values of x, y, z is

(ii) is a right triangle, right angled at Q

…… (i)

is a right triangle right angled at S

is a right triangle right angled at S

…… (ii)

Now substituting in equation (i) we get

Now substituting in equation (iii) we get

Now substituting and in equation (ii) we get

Hence the value of x, y and z are

#### Question 25:

There is a staircase as shown in the given figure, connecting points A and B. Measurements of steps are marked in the figure. Find the straight line distance between A and B.

#### Answer:

We are given the following figure with the related information

In the above figure complete the triangle ABC with right angled at C

So

AC = 2 + 2 + 2 + 2 = 8 and

BC = 1 + 1.6 + 1.6 + 1.8 = 6

Using Pythagoras theorem for triangle ABC to find

Hence the distance between A and B is .

#### Question 26:

In ∆ABC, ∠A = 60°. Prove that BC2 = AB2 + AC2 − AB . AC.

#### Answer:

In ΔABC in which A is an acute angle with 60°.

Now apply Pythagoras theorem in triangle BCD

Hence

#### Question 27:

In ∆ABC, ∠C is an obtuse angle. AD ⊥ BC and AB2 = AC2 + 3 BC2. Prove that BC = CD.

#### Answer:

Given: ΔABC where C is an obtuse angle, AD BC and AB2 = AC2 + 3BC2

To prove: BC = CD

Proof:

In ΔABC,  C is obtuse.

Therefore,

AB2 = AC2 + BC2 + 2BC × DC          (Obtuse angle theorem)          …(1)

AB2 = AC2 + 3BC2         (Given)                …(2)

From (1) and (2), we get

AC2 + 3BC2 = AC2 + BC2 + 2BC × DC

⇒ 3BC2 = BC2 + 2BC × DC

⇒ 2BC2 = 2BC × DC

⇒ BC = DC

#### Question 28:

A point D is on the side BC of an equilateral triangle ABC such that $\mathrm{DC}=\frac{1}{4}\mathrm{BC}$. Prove that AD2 = 13 CD2.

#### Answer:

We are given ABC is an equilateral triangle with

We have to prove

Draw

In and we have

So by right side criterion of similarity we have

Thus we have

and

Since, therefore

We know that AB = BC = AC

We know that

Substitute in we get

Hence we have proved that

#### Question 29:

In ∆ABC, if BD ⊥ AC and BC2 = 2 AC . CD, then prove that AB = AC.

#### Answer:

Since is right triangle right angled at D

In right , we have

Now substitute

#### Question 30:

In a quadrilateral ABCD, given that ∠A + ∠D = 90°. Prove that AC2 + BD2 = AD2 + BC2.

#### Answer:

Given: A quadrilateral ABCD where A + D = 90°.

To prove: AC2 + BD2 = AD2 + BC2

Construction: Extend AB and CD to intersect at O.

Proof:

In ΔAOD, A + O + D = 180°

⇒ ∠O = 90° [A + D = 90°]

Apply Pythagoras Theorem in ΔAOC and ΔBOD,

AC2 = AO2 + OC2

BD2 = OB2 + OD2

∴ AC2 + BD2 = (AO2 + OD2) + (OC2 + OB2)

⇒ AC2 + BD2 = AD2 + BC2

This proves the given relation.

#### Question 31:

In ∆ABC, given that AB = AC and BD AC. Prove that BC2 = 2 AC. CD

#### Answer:

Since is right triangle right angled at D

Substitute

Now, in , we have

Therefore,

Hence proved.

#### Question 32:

ABCD is a rectangle. Points M and N are on BD such that AM ⊥ BD and CN  ⊥ BD. Prove that BM2 + BN2 = DM2 + DN2.

#### Answer:

Given: A rectangle ABCD where AM BD and CN BD.

To prove: BM2 + BN2 = DM2 + DN2

Proof:

Apply Pythagoras Theorem in ΔAMB and ΔCND,

AB2 = AM2 + MB2

CD2 = CN2 + ND2

Since AB = CD, AM2 + MB2 = CN2 + ND2

⇒ AM2 − CN2 = ND2 − MB2 … (i)

Again apply Pythagoras Theorem in ΔAMD and ΔCNB,

AD2 = AM2 + MD2

CB2 = CN2 + NB2

Since AD = BC, AM2 + MD2 = CN2 + NB2

⇒ AM2 − CN2 = NB2 − MD2 … (ii)

Equating (i) and (ii),

ND2 − MB2 = NB2 − MD2

I.e., BM2 + BN2 = DM2 + DN2

This proves the given relation.

#### Question 33:

In ∆ABC, AD is a median. Prove that AB2 + AC2 = 2AD2 + 2DC2.

#### Answer:

We have the following figure.

Since triangle ABM and ACM are right triangles right angled at M

…… (i)

…… (ii)

Adding (i) and (ii), we get

Since in triangle ADM we have

So,

BM + CM = BC

So,

Now we have

So,

Hence proved

#### Question 34:

Prove that in an equilateral triangle, three times the square of a side is equal to four times the square of its altitudes.

#### Answer:

Let ABC be an equilateral triangle and let.

In and we have

And

Since is a right triangle right-angled at D. So

Hence proved.

#### Question 35:

In ∆ABC, ∠ABC = 135°. Prove that AC2 = AB2 + BC2 + 4 ar (∆ABC)

#### Answer:

We have the following figure.

Here is a right triangle right angled at D. Therefore by Pythagoras theorem we have

Again is a right triangle right angled at D.

Therefore, by Pythagoras theorem, we have

Since angle ABD is 45°and therefore angle BAD is also 45°.

Hence AB = DB

So,

Since

So,

Hence we have proved that

#### Question 36:

In a quadrilateral ABCD, ∠B = 90°. If AD2 = AB2 + BC2 + CD2 then prove that ∠ACD = 90°.

#### Answer:

In quadrilateral ABCD, we have

∠B = 90°

So,    (Pythagoras theorem)

and

(Given)

So,

Hence, ∠ACD = 90°                         (Converse of Pythagoras theorem)

#### Question 37:

In a triangle ABC, N is a point on AC such that BN ⊥ AC. If BN2 = AN . NC, prove that ∠B = 90°.

#### Answer:

In , BN ⊥ AC.

Also,

We have to prove that .

In triangles ABN and BNC, we have

Adding above two equations, we get

So,

Hence

#### Question 38:

Nazima is fly fishing in a stream. The tip of her fishing rod is 1.8 m above the surface of the water and the fly at the end of the string rests on the water 3.6 m away and 2.4 m from a point directly under the tip of the road. Assuming that her string (from the tip of her road to the fly) is taut, how much string does she have out (in the given figure)? If she pulls the string at the rate of 5 cm per second, what will the horizontal distance of the fly from her after 12 seconds.

#### Answer:

Let us take

AB = Tip of fishing rod above the surface of the water

BC = The string rest from directly under the tip of the rod.

AC = The length of string

In ABC right triangle right angled at B

Hence 3 m string is out

The string pulled in 12 seconds is equal to to point

In this case the diagram will look like the following

Now The length of the new string AD = ACCD = 3.00 − 0.6 = 2.4 m

Now in triangle ADB we have

∴ Required distance =

Hence, the horizontal distance is .

#### Question 1:

State basic proportionality theorem and its converse.

#### Answer:

TO STATE: The basic proportionality theorem and its converse.

BASIC PROPORTIONALITY THEOREM: If a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

CONVERSE OF BASIC PROPORTIONALITY THEOREM: If a line divides any two sides of a triangle in the same ratio, then the line must be parallel to the third side.

#### Question 2:

In the adjoining figure, find AC.

#### Answer:

GIVEN: In the figure we are given AD = 6cm, BD = 9cm, AE = 8cm

TO FIND: AC

According to BASIC PROPORTIONALITY THEOREM, if a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

In ∆ABC, DE || BC. So,

Now,

#### Question 3:

In the adjoining figure, if AD is the bisector of ∠A, what is AC?

#### Answer:

GIVEN: AB = 6cm, BD = 3cm and DC = 2cm. Also, AD is the bisector of .

TO FIND: AC

SOLUTION: We know that the internal bisector of a triangle divides the opposite side internally in the ratio of the sides containing the angle. Therefore,

#### Question 17:

In the given figure, PQ || BC and AP : PB = 1 : 2. Find .

#### Answer:

GIVEN: In the given figure PQ || BC, and AP: PB = 1:2

TO FIND:

We know that according to basic proportionality theorem if a line is drawn parallel to one side of a triangle intersecting the other side, then it divides the two sides in the same ratio.

Since triangle APQ and ABC are similar

Hence,

Now, it is given that .

So;

Since the ratio of the areas of two similar triangle is equal to the ratio of the squares of their corresponding sides.

$\frac{\mathrm{Area}\left(\mathrm{APQ}\right)}{\mathrm{Area}\left(\mathrm{ABC}\right)}={\left(\frac{\mathrm{AP}}{\mathrm{AB}}\right)}^{2}={\left(\frac{1}{3}\right)}^{2}=\frac{1}{9}$

Hence we got the result

#### Question 4:

State AAA similarity criterion.

#### Answer:

AAA Similarity Criterion: If two triangles are equiangular, then they are similar.

In ΔABC and ΔDEF, if

Then,

#### Question 5:

State SSS similarity criterion.

#### Answer:

SSS Similarity Criterion: If the corresponding sides of two triangles are proportional, then they are similar.

In ΔABC and ΔDEF, if

Then,

#### Question 6:

State SAS similarity criterion.

#### Answer:

SAS Similarity Criterion: If in two triangles, one pair of corresponding sides are proportional and the included angles are equal then two triangles are similar.

In ΔABC and ΔDEF, if

Then,

#### Question 7:

In the adjoining figure, DE is parallel to BC and AD = 1 cm, BD = 2 cm. What is the ratio of the area of ∆ABC to the area of ∆ADE?

#### Answer:

GIVEN: DE is parallel to BC, AD = 1cm and BD = 2cm.

TO FIND: Ratio of ΔABC to area of ΔADE

According to BASIC PROPORTIONALITY THEOREM, if a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

In ΔABC, DE || BC.

So

#### Question 8:

In the figure given below DE || BC. If AD = 2.4 cm, DB = 3.6 cm, AC = 5 cm. Find AE.

#### Answer:

GIVEN: AD = 2.4cm, BD = 3.6cm and AC = 5cm.

TO FIND: AE

According to BASIC PROPORTIONALITY THEOREM If a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

In ∆ABC, DE || BC.

#### Question 9:

If the areas of two similar triangles ABC and PQR are in the ratio 9 : 16 and BC = 4.5 cm, what is the length of QR?

#### Answer:

Given:  ΔABC and ΔPQR are similar triangles. Area of ΔABC: Area of ΔPQR = 9:16 and BC = 4.5cm.

To find: Length of QR

We know that the ratio of the areas of two similar triangles is equal to the ratio of the squares of any two corresponding sides.

Hence,

#### Question 10:

The areas of two similar triangles are 169 cm2 and 121 cm2 respectively. If the longest side of the larger triangle is 26 cm, what is the length of the longest side of the smaller triangle?

#### Answer:

Let ∆ABC and ΔPQR are similar triangles. The area of triangles is 169cm2 and 121cm2, respectively.

Longest side of the larger triangle is 26cm

TO FIND: length of longest side of the smaller side.

Suppose longest side of the larger triangle is BC and longest side of the smaller triangle is QR.

We know that the ratio of the areas of two similar triangles is equal to the ratio of the squares of any two corresponding sides.

Hence,

#### Question 11:

If ABC and DEF are similar triangles such that ∠A = 57° and ∠E = 73°, what is the measure of ∠C?

#### Answer:

GIVEN: There are two similar triangles ΔABC and ΔDEF.

,

TO FIND: measure of

SAS Similarity Criterion: If in two triangles, one pair of corresponding sides are proportional and the included angles are equal then two triangles are similar.

In ΔABC and ΔDEF if

Then,

So,

Similarly

Now we know that sum of all angles of a triangle is equal to 180°,

#### Question 12:

If the altitude of two similar triangles are in the ratio 2 : 3, what is the ratio of their areas?

#### Answer:

GIVEN: Altitudes of two similar triangles are in ratio 2:3.

TO FIND: Ratio of the areas of two similar triangles.

Let first triangle be ΔABC and the second triangle be ΔPQR

We know that the areas of two similar triangles are in the ratio of the squares of the corresponding altitudes.

#### Question 13:

If ∆ABC and ∆DEF are two triangles such that $\frac{\mathrm{AB}}{\mathrm{DE}}=\frac{\mathrm{BC}}{\mathrm{EF}}=\frac{\mathrm{CA}}{\mathrm{FD}}=\frac{3}{4}$, then write Area (∆ABC) : Area (∆DEF)

#### Answer:

GIVEN: ΔABC and ΔDEF are two triangles such that .

TO FIND:

We know that two triangles are similar if their corresponding sides are proportional.

Here, ΔABC and ΔDEF are similar triangles because their corresponding sides are given proportional, i.e.

Since the ratio of the areas of two similar triangle is equal to the ratio of the squares of their corresponding sides.

#### Question 14:

If ∆ABC and ∆DEF are similar triangles such that AB = 3 cm, BC = 2 cm, CA = 2.5 cm and EF = 4 cm, write the perimeter of ∆DEF.

#### Answer:

GIVEN: ΔABC and ΔDEF are similar triangles such that AB = 3cm, BC = 2cm, CA = 2.5cm and EF = 4cm.

TO FIND: Perimeter of ΔDEF.

We know that if two triangles are similar then their corresponding sides are proportional.

Hence,

Substituting the values, we get

Similarly,

#### Question 15:

State Pythagoras theorem and its converse.

#### Answer:

TO STATE: Pythagoras theorem and its converse.

PYTHAGORAS THEOREM: In a right angled triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides.

CONVERSE OF PYTHAGORAS THEOREM: In a triangle, if the square of one side is equal to the sum of the square of the other two sides then the angle opposite to the greatest side is a right angle.

#### Question 16:

The lengths of the diagonals of a rhombus are 30 cm and 40 cm. Find the side of the rhombus.

#### Answer:

GIVEN: the lengths of the diagonals of a rhombus are 30 cm and 40 cm.

TO FIND: side of the rhombus.

Let the diagonals AC and CD of the rhombus ABCD meet at point O.

We know that the diagonals of the rhombus bisect each other perpendicularly.

Hence in right triangle AOD, by Pythagoras theorem

Hence the side of the rhombus is

#### Question 18:

In the given figure, LM = LN = 46°. Express x in terms of a, b and c where a, b, c are lengths of LM, MN and NK respectively.

#### Answer:

Given: In the given figure

TO EXPRESS: x in terms of a, b, c where a, b, and c are the lengths of LM, MN and NK respectively.

Here we can see that. It forms a pair of corresponding angles.

Hence, LM || PN

In $∆\mathrm{LMK}$ and $∆\mathrm{PNK}$,

$\frac{\mathrm{ML}}{\mathrm{NP}}=\frac{\mathrm{MK}}{\mathrm{NK}}\phantom{\rule{0ex}{0ex}}\frac{a}{x}=\frac{b+c}{c}\phantom{\rule{0ex}{0ex}}x=\frac{ac}{b+c}$

Hence we got the result as .

#### Question 19:

In the given figure, S and T are points on the sides PQ and PR respectively of ∆PQR such that PT = 2 cm, TR = 4 cm and ST is parallel to QR. Find the ratio of the areas of ∆PST and ∆PQR.

#### Answer:

Given: In ΔPQR, S and T are the points on the sides PQ and PR respectively such that PT = 2cm, TR = 4cm and ST is parallel to QR.

To find: Ratio of areas of ΔPST and ΔPQR

Now, we know that the areas of two similar triangles are in the ratio of the squares of the corresponding sides. Therefore,

#### Question 20:

In the given figure, ∆AHK is similar to ∆ABC. If AK = 10 cm, BC = 3.5 cm and HK = 7 cm, find AC.

#### Answer:

Given:

AK = 10 cm

BC = 3.5 cm

HK = 7 cm

To find: AC

Since, so their corresponding sides are proportional.

#### Question 21:

In the given figure, DE || BC in ∆ABC such that BC = 8 cm, AB = 6 cm and DA = 1.5 cm. Find DE.

#### Answer:

Given: In ∆ABC, DE || BC. BC = 8 cm, AB = 6 cm and DA = 1.5 cm.

To find: DE

In ∆ABC and ∆ADE

So,

#### Question 22:

In the given figure, DE || BC and $\mathrm{AD}=\frac{1}{2}\mathrm{BD}$. If BC = 4.5 cm, find DE.

#### Answer:

Given: In ∆ABC, DE || BC. and BC = 4.5 cm.

To find: DE

In ∆ABC and ∆ADE

#### Question 1:

A vertical stick 20 m long casts a shadow 10 m long on the ground. At the same time, a tower casts a shadow 50 m long on the ground. The height of the tower is

(a) 100 m
(b) 120 m
(c) 25 m
(d) 200 m

#### Answer:

Given: Vertical stick 20m long casts a shadow 10m long on the ground. At the same time a tower casts the shadow 50 m long on the ground.

To determine: Height of the tower

Let AB be the vertical stick and AC be its shadow. Also, let DE be the vertical tower and DF be its shadow.

Join BC and EF.

In ΔABC and ΔDEF, we have

We know that in any two similar triangles, the corresponding sides are proportional. Hence,

Hence the correct answer is option .

#### Question 2:

Sides of two similar triangles are in the ratio 4 : 9. Areas of these triangles are in the ratio.

(a) 2 : 3
(b) 4 : 9
(c) 81 : 16
(d) 16 : 81

#### Answer:

Given: Sides of two similar triangles are in the ratio 4:9

To find: Ratio of area of these triangles

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence the correct answer is option

#### Question 3:

The areas of two similar triangles are in respectively 9 cm2 and 16 cm2. The ratio of their corresponding sides is

(a) 3 : 4
(b) 4 : 3
(c) 2 : 3
(d) 4 : 5

#### Answer:

Given: Areas of two similar triangles are 9cm2 and 16cm2.

To find: Ratio of their corresponding sides.

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Taking square root on both sides, we get

$\frac{\mathrm{side}1}{\mathrm{side}2}=\frac{3}{4}$

So, the ratio of their corresponding sides is 3 : 4.

Hence the correct answer is .

#### Question 4:

The areas of two similar triangles ∆ABC and ∆DEF are 144 cm2 and 81 cm2 respectively. If the longest side of larger ∆ABC be 36 cm, then the longest side of the smaller triangle ∆DEF is

(a) 20 cm
(b) 26 cm
(c) 27 cm
(d) 30 cm

#### Answer:

Given: Areas of two similar triangles ΔABC and ΔDEF are 144cm2 and 81cm2.

If the longest side of larger ΔABC is 36cm

To find: the longest side of the smaller triangle ΔDEF

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Taking square root on both sides, we get

= 27 cm

Hence the correct answer is

#### Question 5:

∆ABC and ∆BDE are two equilateral triangles such that D is the mid-point of BC. The ratio of the areas of triangle ABC and BDE is

(a) 2 : 1
(b) 1 : 2
(c) 4 : 1
(d) 1 : 4

#### Answer:

Given: ΔABC and ΔBDE are two equilateral triangles such that D is the midpoint of BC.

To find: Ratio of areas of ΔABC and ΔBDE.

ΔABC and ΔBDE are equilateral triangles; hence they are similar triangles.

Since D is the midpoint of BC, BD = DC.

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence the correct answer is .

#### Question 6:

Two isosceles triangles have equal angles and their areas are in the ratio 16 : 25. The ratio of their corresponding heights is

(a) 4 : 5
(b) 5 : 4
(c) 3 : 2
(d) 5 : 7

#### Answer:

Given: Two isosceles triangles have equal vertical angles and their areas are in the ratio of 16:25.

To find: Ratio of their corresponding heights.

Let ∆ABC and ∆PQR be two isosceles triangles such that $\angle \mathrm{A}=\angle \mathrm{P}$. Suppose AD ⊥ BC and PS ⊥ QR .

In ∆ABC and ∆PQR,

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding altitudes.

Hence,

$\frac{\mathrm{Ar}\left(∆\mathrm{ABC}\right)}{\mathrm{Ar}\left(∆\mathrm{PQR}\right)}={\left(\frac{\mathrm{AD}}{\mathrm{PS}}\right)}^{2}\phantom{\rule{0ex}{0ex}}⇒\frac{16}{25}={\left(\frac{\mathrm{AD}}{\mathrm{PS}}\right)}^{2}\phantom{\rule{0ex}{0ex}}⇒\frac{\mathrm{AD}}{\mathrm{PS}}=\frac{4}{5}$

Hence we got the result as

#### Question 7:

If ∆ABC and ∆DEF are similar such that 2AB = DE and BC = 8 cm, then EF =

(a) 16 cm
(b) 12 cm
(c) 8 cm
(d) 4 cm

#### Answer:

Given: ΔABC and ΔDEF are similar triangles such that 2AB = DE and BC = 8 cm.

To find: EF

We know that if two triangles are similar then there sides are proportional.

Hence, for similar triangles ΔABC and ΔDEF

Hence the correct answer is .

#### Question 8:

If ∆ABC and ∆DEF are two triangles such that $\frac{\mathrm{AB}}{\mathrm{DE}}=\frac{\mathrm{BC}}{\mathrm{EF}}=\frac{\mathrm{CA}}{\mathrm{FD}}=\frac{2}{5}$, then Area (∆ABC) : Area (∆DEF) =

(a) 2 : 5
(b) 4 : 25
(c) 4 : 15
(d) 8 : 125

#### Answer:

Given: ΔABC and ΔDEF are two triangles such that .

To find:

We know that if the sides of two triangles are proportional, then the two triangles are similar.

Since , therefore, ΔABC and ΔDEF are similar.

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence the correct answer is .

#### Question 9:

∆ABC is such that AB = 3 cm, BC = 2 cm and CA = 2.5 cm. If ∆DEF ∼ ∆ABC and EF = 4 cm, then perimeter of ∆DEF is

(a) 7.5 cm
(b) 15 cm
(c) 22.5 cm
(d) 30 cm

#### Answer:

Given: In ΔABC, AB = 3cm, BC = 2cm, CA = 2.5cm. and EF = 4cm.

To find: Perimeter of ΔDEF.

We know that if two triangles are similar, then their sides are proportional

Since ΔABC and ΔDEF are similar,

From (1) and (2), we get

Perimeter of ΔDEF =  DE + EF + FD = 6 + 4 +5 = 15 cm

Hence the correct answer is .

#### Question 10:

XY is drawn parallel to the base BC of a ∆ABC cutting AB at X and AC at Y. If AB = 4 BX and YC = 2 cm, then AY =

(a) 2 cm
(b) 4 cm
(c) 6 cm
(d) 8 cm

#### Answer:

Given: XY is drawn parallel to the base BC of a ΔABC cutting AB at X and AC at Y. AB = 4BX and YC = 2 cm.

To find: AY

In ΔAXY and ΔABC,

We know that if two triangles are similar, then their sides are proportional.

It is given that AB = 4BX.

Let AB = 4x and BX = x.

Then, AX = 3x

Hence the correct answer is .

#### Question 11:

Two poles of height 6 m and 11 m stand vertically upright on a plane ground. If the distance between their foot is 12 m, the distance between their tops is

(a) 12 m
(b) 14 m
(c) 13 m
(d) 11 m

#### Answer:

Given: Two poles of heights 6m and 11m stand vertically upright on a plane ground. Distance between their foot is 12 m.

To find: Distance between their tops.

Let CD be the pole with height 6m.

AB is the pole with height 11m, distance between their foot i.e. DB is 12 m.

Let us assume a point E on the pole AB which is 6m from the base of AB.

Hence

AE = AB − 6 = 11 − 6 = 5 m

Now in right triangle AEC, Applying Pythagoras theorem

AC2 = AE2 + EC2

AC2 = 52 + 122                  (since CDEB forms a rectangle and opposite sides of rectangle are equal)

AC2 = 25 + 144

AC2 = 169

Thus, the distance between their tops is 13m.

Hence correct answer is .

#### Question 12:

In ∆ABC, a line XY parallel to BC cuts AB at X and AC at Y. If BY bisects ∠XYC, then

(a) BC = CY
(b) BC = BY
(c) BC ≠ CY
(d) BC ≠ BY

#### Answer:

Given: XY||BC and BY is bisector of $\angle$XYC.

Since XY||BC

So $\angle$YBC = $\angle$BYC     (Alternate angles)

Now, in triangle BYC two angles are equal. Therefore, the two corresponding sides will be equal.

Hence, BC = CY

Hence option (a) is correct.

#### Question 13:

In ∆ABC, D and E are points on side AB and AC respectively such that DE || BC and AD : DB = 3 : 1. If EA = 3.3 cm, then AC =

(a) 1.1 cm
(b) 4 cm
(c) 4.4 cm
(d) 5.5 cm

#### Answer:

Given: In ΔABC, D and E are points on the side AB and AC respectively such that DE || BC and AD : DB = 3 : 1. Also, EA = 3.3cm.

To find: AC

In ∆ABC, DE || BC.

Using corollory of basic proportionality theorem, we have

Hence the correct answer is .

#### Question 14:

In triangles ABC and DEF, ∠A = ∠E = 40°, AB : ED = AC : EF and ∠F = 65°, then ∠B =

(a) 35°
(b) 65°
(c) 75°
(d) 85°

#### Answer:

Given: In ΔABC and ΔDEF

To find: Measure of angle B.

In ΔABC and ΔDEF

Hence in similar triangles ΔABC and ΔDEF

We know that sum of all the angles of a triangle is equal to 180°.

Hence the correct answer is .

#### Question 15:

If ABC and DEF are similar triangles such that ∠A = 47° and ∠E = 83°, then ∠C =

(a) 50°
(b) 60°
(c) 70°
(d) 80°

#### Answer:

Given: If ΔABC and ΔDEF are similar triangles such that

To find: Measure of angle C

In similar ΔABC and ΔDEF,

We know that sum of all the angles of a triangle is equal to 180°.

Hence the correct answer is

#### Question 16:

If D, E, F are the mid-points of sides BC, CA and AB respectively of ∆ABC, then the ratio of the areas of triangles DEF and ABC is

(a) 1 : 4
(b) 1 : 2
(c) 2 : 3
(d) 4 : 5

#### Answer:

GIVEN: In ΔABC, D, E and F are the midpoints of BC, CA, and AB respectively.

TO FIND: Ratio of the areas of ΔDEF and ΔABC

Since it is given that D and, E are the midpoints of BC, and AC respectively.

Therefore DE || AB, DE || FA……(1)

Again it is given that D and, F are the midpoints of BC, and, AB respectively.

Therefore, DF || CA, DF || AE……(2)

From (1) and (2) we get AFDE is a parallelogram.

Similarly we can prove that BDEF is a parallelogram.

Now, in ΔADE and ΔABC

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence the correct option is .

#### Question 24:

ABCD is a trapezium such that BC || AD and AD = 4 cm. If the diagonals AC and BD intersect at O such that $\frac{\mathrm{AO}}{\mathrm{OC}}=\frac{\mathrm{DO}}{\mathrm{OB}}=\frac{1}{2}$, then BC =

(a) 7 cm
(b) 8 cm
(c) 9 cm
(d) 6 cm

#### Answer:

Given: ABCD is a trapezium in which BC||AD and AD = 4 cm

The diagonals AC and BD intersect at O such that

To find: DC

In ΔAOD and ΔCOB

Hence the correct answer is

#### Question 17:

In a ∆ABC, ∠A = 90°, AB = 5 cm and AC = 12 cm. If AD ⊥ BC, then AD =

(a) $\frac{13}{2}\mathrm{cm}$
(b) $\frac{60}{13}\mathrm{cm}$
(c) $\frac{13}{60}\mathrm{cm}$
(d) $\frac{2\sqrt{15}}{13}\mathrm{cm}$

#### Answer:

Given: In ΔABC,, AC = 12cm, and AB = 5cm.

To find: AD

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

In ∆ACB and ∆ADC,

$\angle \mathrm{C}=\angle \mathrm{C}$        (Common)

$\angle \mathrm{A}=\angle \mathrm{ADC}=90°$

∴ ∆ACB $~$∆ADC     (AA Similarity)

We got the result as

#### Question 18:

In an equilateral triangle ABC, If AD ⊥ BC, then

(a) 2AB2 = 3AD2
(b) 4AB2 = 3AD2
(c) 3AB2 = 4AD2
(d) 3AB2 = 2AD2

#### Answer:

Given: In an equilateral ΔABC,

Applying Pythagoras theorem,

In ΔABD,

We got the result as .

#### Question 19:

If ∆ABC is an equilateral triangle such that AD ⊥ BC, then AD2 =

(a) $\frac{3}{2}{\mathrm{DC}}^{2}$
(b) 2 DC2
(c) 3 CD2
(d) 4 DC2

#### Answer:

Given: In an equilateral ΔABC, .

Since , BD = CD = $\frac{\mathrm{BC}}{2}$

Applying Pythagoras theorem,

In ΔADC

We got the result as

#### Question 20:

In a ∆ABC, perpendicular AD from A and BC meets BC at D. If BD = 8 cm, DC = 2 cm and AD = 4 cm, then

(a) ∆ABC is isosceles
(b) ∆ABC is equilateral
(c) AC = 2AB
(d) ∆ABC is right-angled at A

#### Answer:

Given: In ΔABC,, BD = 8cm, DC = 2 cm and AD = 4cm.

In ΔADC,

Similarly, in ΔADB

Now, In ΔABC

${\mathrm{BC}}^{2}={\left(\mathrm{CD}+\mathrm{DB}\right)}^{2}={\left(2+8\right)}^{2}={\left(10\right)}^{2}=100$

and

${\mathrm{AB}}^{2}+{\mathrm{CA}}^{2}=80+20=100$

$\therefore {\mathrm{AB}}^{2}+{\mathrm{CA}}^{2}={\mathrm{BC}}^{2}$

Hence, triangle ABC is right angled at A.

We got the result as

#### Question 21:

In a ∆ABC, point D is on side AB and point E is on side AC, such that BCED is a trapezium. If DE : BC = 3 : 5, then Area (∆ ADE) : Area (◻BCED) =

(a) 3 : 4
(b) 9 : 16
(c) 3 : 5
(d) 9 : 25

#### Answer:

Given: In ΔABC, D is on side AB and point E is on side AC, such that BCED is a trapezium. DE: BC = 3:5.

To find: Calculate the ratio of the areas of ΔADE and the trapezium BCED.

In ΔADE and ΔABC,

We know that

Let Area of ΔADE = 9x sq. units and Area of ΔABC = 25x sq. units

Now ,

Hence the correct answer is.

#### Question 22:

In a ∆ABC, AD is the bisector of ∠BAC. If AB = 6 cm, AC = 5 cm and BD = 3 cm, then DC =

(a) 11.3 cm
(b) 2.5 cm
(c) 3 : 5 cm
(d) None of these

#### Answer:

Given: In a ΔABC, AD is the bisector of . AB = 6cm and AC = 5cm and BD = 3cm.

To find: DC

We know that the internal bisector of angle of a triangle divides the opposite side internally in the ratio of the sides containing the angle.

Hence,

Hence we got the result

#### Question 23:

In a ∆ABC, AD is the bisector of ∠BAC. If AB = 8 cm, BD = 6 cm and DC = 3 cm. Find AC

(a) 4 cm
(b) 6 cm
(c) 3 cm
(d) 8 cm

#### Answer:

Given: In a ΔABC, AD is the bisector of angle BAC. AB = 8cm, and DC = 3cm and BD = 6cm.

To find: AC

We know that the internal bisector of angle of a triangle divides the opposite side internally in the ratio of the sides containing the angle.

Hence,

Hence we got the result

#### Question 25:

If ABC is an isosceles triangle and D is a point of BC such that AD ⊥ BC, then

(a) AB2 − AD2 = BD.DC
(b) AB2 − AD2 = BD2 − DC2
(c) AB2 + AD2 = BD.DC
(d) AB2 + AD2 = BD2 − DC2

#### Answer:

Given: ΔABC is an isosceles triangle, D is a point on BC such that

We know that in an isosceles triangle the perpendicular from the vertex bisects the base.

∴ BD = DC

Applying Pythagoras theorem in ΔABD

Since

Hence correct answer is .

#### Question 26:

∆ABC is a right triangle right-angled at A and  AD ⊥ BC. Then, $\frac{\mathrm{BD}}{\mathrm{DC}}=$
(a) ${\left(\frac{\mathrm{AB}}{\mathrm{AC}}\right)}^{2}$
(b) $\frac{\mathrm{AB}}{\mathrm{AC}}$
(c) ${\left(\frac{\mathrm{AB}}{\mathrm{AD}}\right)}^{2}$
(d) $\frac{\mathrm{AB}}{\mathrm{AD}}$

#### Answer:

Given: In ΔABC, and.

To find: BD: DC

In ΔADB and ΔADC,

Disclaimer: The question is not correct. The given ratio cannot be evaluated using the given conditions in the question.

#### Question 27:

If ABC is a right triangle right-angled at B and M, N are the mid-points of AB and BC respectively, then 4(AN2 + CM2) =

(a) 4 AC2
(b) 5 AC2
(c) $\frac{5}{4}{\mathrm{AC}}^{2}$
(d) 6 AC2

#### Answer:

M is the mid-point of AB.

$\mathrm{BM}=\frac{\mathrm{AB}}{2}$

N is the mid-point of BC.

$\mathrm{BN}=\frac{\mathrm{BC}}{2}$

Now,

Hence option (b) is correct.

#### Question 28:

If E is a point on side CA of an equilateral triangle ABC such that BE ⊥ CA, then AB2 + BC2 + CA2 =

(a) 2 BE2
(b) 3 BE2
(c) 4 BE2
(d) 6 BE2

#### Answer:

In triangle ABC, E is a point on AC such that .

We need to find .

Since , CE = AE = $\frac{\mathit{A}\mathit{C}}{\mathit{2}}$              (In a equilateral triangle, the perpendicular from the vertex bisects the base.)

In triangle ABE, we have

Since AB = BC = AC

Therefore,

Since in triangle BE is an altitude, so

Hence option (c) is correct.

#### Question 29:

In a right triangle ABC right-angled at B, if P and Q are points on the sides AB and AC respectively, then

(a) AQ2 + CP2 = 2(AC2 + PQ2)
(b) 2(AQ2 + CP2) = AC2 + PQ2
(c) AQ2 + CP2 = AC2 + PQ2
(d) $\mathrm{AQ}+\mathrm{CP}=\frac{1}{2}\left(\mathrm{AC}+\mathrm{PQ}\right)$

#### Answer:

Disclaimer: There is mistake in the problem. The question should be "In a right triangle ABC right-angled at B, if P and Q are points on the sides AB and BC respectively, then"

Given: In the right ΔABC, right angled at B. P and Q are points on the sides AB and BC respectively.

Applying Pythagoras theorem,

In ΔAQB,

${\mathrm{AQ}}^{2}={\mathrm{AB}}^{2}+{\mathrm{BQ}}^{2}$       .....(1)

In ΔPBC

.....(2)

Adding (1) and (2), we get

${\mathrm{AQ}}^{2}+{\mathrm{CP}}^{2}={\mathrm{AB}}^{2}+{\mathrm{BQ}}^{2}+{\mathrm{PB}}^{2}+{\mathrm{BC}}^{2}\phantom{\rule{0ex}{0ex}}$    .....(3)

In ΔABC,

.....(4)

In ΔPBQ,

${\mathrm{PQ}}^{2}={\mathrm{PB}}^{2}+{\mathrm{BQ}}^{2}$        .....(5)

From (3), (4) and (5), we get

${\mathrm{AQ}}^{2}+{\mathrm{CP}}^{2}={\mathrm{AC}}^{2}+{\mathrm{PQ}}^{2}$

We got the result as

#### Question 30:

If in ∆ABC and ∆DEF, $\frac{\mathrm{AB}}{\mathrm{DE}}=\frac{\mathrm{BC}}{\mathrm{FD}}$, then ∆ABC ∼ ∆DEF when

(a) ∠A = ∠F
(b) ∠A = ∠D
(c) ∠B = ∠D
(d) ∠B = ∠E

#### Answer:

Given: In ΔABC and ΔDEF, .

We know that if in two triangles, one pair of corresponding sides are proportional and the included angles are equal, then the two triangles are similar.

Then,

Hence, ΔABC is similar to ΔDEF, we should have.

Hence the correct answer is .

#### Question 31:

If in two triangles ABC and DEF, $\frac{\mathrm{AB}}{\mathrm{DE}}=\frac{\mathrm{BC}}{\mathrm{FE}}=\frac{\mathrm{CA}}{\mathrm{FD}}$, then

(a) ∆FDE ∼ ∆CAB
(b) ∆FDE ∼ ∆ABC
(c) ∆CBA ∼ ∆FDE
(d) ∆BCA ∼ ∆FDE

#### Answer:

We know that if two triangles are similar if their corresponding sides are proportional.

It is given that ΔABC and ΔDEF are two triangles such that .

$\angle A=\angle D\phantom{\rule{0ex}{0ex}}\angle B=\angle E\phantom{\rule{0ex}{0ex}}\angle C=\angle F\phantom{\rule{0ex}{0ex}}$

∴ ΔCAB $~$ΔFDE

Hence the correct answer is .

#### Question 32:

∆ABC ∼ ∆DEF, ar(∆ABC) = 9 cm2, ar(∆DEF) = 16 cm2. If BC = 2.1 cm, then the measure of EF is

(a) 2.8 cm
(b) 4.2 cm
(c) 2.5 cm
(d) 4.1 cm

#### Answer:

Given:

To find: measure of EF

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence the correct answer is

#### Question 33:

The length of the hypotenuse of an isosceles right triangle whose one side is is

(a) 12 cm
(b) 8 cm
(c)
(d)

#### Answer:

Given: One side of isosceles right triangle is 4√2cm

To find: Length of the hypotenuse.

We know that in isosceles triangle two sides are equal.

In isosceles right triangle ABC, let AB and AC be the two equal sides of measure 4√2cm.

Applying Pythagoras theorem, we get

Hence correct answer is .

#### Question 34:

A man goes 24 m due west and then 7 m due north. How far is he from the starting point?

(a) 31 m
(b) 17 m
(c) 25 m
(d) 26 m

#### Answer:

A man goes 24m due to west and then 7m due north.

Let the man starts from point B and goes 24 m due to west and reaches point A, then walked 7m north and reaches point C.

Now we have to find the distance between the starting point and the end point i.e. BC.

In right triangle ABC, applying Pythagoras theorem, we get

Hence correct answer is .

#### Question 35:

∆ABC ∼ ∆DEF. If BC = 3 cm, EF = 4 cm and ar(∆ABC) = 54 cm2, then ar(∆DEF) =

(a) 108 cm2
(b) 96 cm2
(c) 48 cm2
(d) 100 cm2

#### Answer:

Given: In Δ ABC and Δ DEF

To find: Ar(Δ DEF)

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence the correct answer is

#### Question 36:

∆ABC ∼ ∆PQR such that ar(∆ABC) = 4 ar(∆PQR). If BC = 12 cm, then QR =

(a) 9 cm
(b) 10 cm
(c) 6 cm
(d) 8 cm

#### Answer:

Given: In Δ ABC and ΔPQR

To find: Measure of QR

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence the correct answer is

#### Question 37:

The areas of two similar triangles are 121 cm2 and 64 cm2 respectively. If the median of the first triangle is 12.1 cm, then the corresponding median of the other triangle is

(a) 11 cm
(b) 8.8 cm
(c) 11.1 cm
(d) 8.1 cm

#### Answer:

Given: The area of two similar triangles is 121cm2 and 64cm2 respectively. The median of the first triangle is 2.1cm.

To find: Corresponding medians of the other triangle

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their medians.

Taking square root on both side, we get

Hence the correct answer is .

#### Question 38:

If ∆ABC ∼ ∆DEF such that DE = 3 cm, EF = 2 cm, DF = 2.5 cm, BC = 4 cm, then perimeter of ∆ABC is

(a) 18 cm
(b) 20 cm
(c) 12 cm
(d) 15 cm

#### Answer:

Given: ΔABC and ΔDEF are similar triangles such that DE = 3cm, EF = 2cm, DF = 2.5cm and BC = 4cm.

To find: Perimeter of ΔABC.

We know that if two triangles are similar then their corresponding sides are proportional.

Hence,

Substituting the values we get

Similarly,

Hence the correct option is

#### Question 39:

In an equilateral triangle ABC if AD ⊥ BC, then AD2 =

(a) CD2
(b) 2CD2
(c) 3CD2
(d) 4CD2

#### Answer:

In an equilateral ΔABC, .

In ΔADC, applying Pythagoras theorem, we get

Hence, the correct option is (c).

#### Question 40:

In an equilateral triangle ABC if AD ⊥ BC, then

(a) 5AB2 = 4AD2
(b) 3AB2 = 4AD2
(c) 4AB2 = 3AD2
(d) 2AB2 = 3AD2

#### Answer:

∆ABC is an equilateral triangle and .

In ∆ABD, applying Pythagoras theorem, we get

We got the result as .

#### Question 41:

If ∆ABC ∼ ∆DEF such that AB = 9.1 cm and DE = 6.5 cm. If the perimeter of ∆DEF is 25 cm, then the perimeter of ∆ABC is

(a) 36 cm
(b) 30 cm
(c) 34 cm
(d) 35 cm

#### Answer:

Given: ΔABC is similar to ΔDEF such that AB= 9.1cm, DE = 6.5cm. Perimeter of ΔDEF is 25cm.

To find: Perimeter of ΔABC.

We know that the ratio of corresponding sides of similar triangles is equal to the ratio of their perimeters.

Hence,

Hence the correct answer is .

#### Question 42:

In an isosceles triangle ABC if AC = BC and AB2 = 2AC2, then ∠C =

(a) 30°
(b) 45°
(c) 90°
(d) 60°

#### Answer:

Given: In Isosceles ΔABC, AC = BC and AB2 = 2AC2.

To find: Measure of angle C

In Isosceles ΔABC,

AC = BC

$\angle \mathrm{B}=\angle \mathrm{A}$    (Equal sides have equal angles opposite to them)

Hence the correct answer is .

#### Question 43:

∆ABC is an isosceles triangle in which ∠C = 90. If AC = 6 cm, then AB =

(a)
(b) 6 cm
(c)
(d)

#### Answer:

Given: In an isosceles ΔABC, , AC = 6 cm.

To find: AB

In an isosceles ΔABC,.

Therefore, BC = AC = 6 cm

Applying Pythagoras theorem in ΔABC, we get

We got the result as .

#### Question 44:

If in two triangle ABC and DEF, ∠A = ∠E, ∠B = ∠F, then which of the following is not true?

(a) $\frac{\mathrm{BC}}{\mathrm{DF}}=\frac{\mathrm{AC}}{\mathrm{DE}}$
(b) $\frac{\mathrm{AB}}{\mathrm{DE}}=\frac{\mathrm{BC}}{\mathrm{DF}}$
(c) $\frac{\mathrm{AB}}{\mathrm{EF}}=\frac{\mathrm{AC}}{\mathrm{DE}}$
(d) $\frac{\mathrm{BC}}{\mathrm{DF}}=\frac{\mathrm{AB}}{\mathrm{EF}}$

#### Answer:

In ΔABC and ΔDEF

∴ ΔABC and ΔDEF are similar triangles.

Hence

Hence the correct answer is (b).

#### Question 45:

In an isosceles triangle ABC, if AB = AC = 25 cm and BC = 14 cm, the measure of altitude from A on BC is

(a) 20 cm
(b) 22 cm
(c) 18 cm
(d) 24 cm

#### Answer:

Given: In an isosceles triangle ABC, AB = AC = 25 cm and BC = 14cm.

To find: Measure of altitude from A on BC

Since AD ⊥ BC, so BD = CD =

In right ∆ABD,

We got the result as .

#### Question 46:

In the given figure the measure of ∠D and ∠F are respectively

(a) 50°, 40°
(b) 20°, 30°
(c) 40°, 50°
(d) 30°, 20°

#### Answer:

ΔABC and ΔDEF,

$\frac{\mathrm{AB}}{\mathrm{AC}}=\frac{\mathrm{EF}}{\mathrm{ED}}\phantom{\rule{0ex}{0ex}}\angle \mathrm{A}=\angle \mathrm{E}=130°$

ΔABC $~$ ΔEFD   (SAS Similarity)

$\therefore \angle \mathrm{F}=\angle \mathrm{B}=30°\phantom{\rule{0ex}{0ex}}\angle \mathrm{D}=\angle \mathrm{C}=20°$

Hence the correct answer is

#### Question 47:

In the given figure, the value of x for which DE || AB is

(a) 4
(b) 1
(c) 3
(d) 2

#### Answer:

Given: In ∆ABC,  DE || AB.

To find: the value of x

According to basic proportionality theorem if a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

In ∆ABC,  DE || AB

x = 2

Hence we got the result .

#### Question 48:

In the given figure, if ∠ADE = ∠ABC, then CE =

(a) 2
(b) 5
(c) 9/2
(d) 3

#### Answer:

Given:

To find: The value of CE

Since

∴ DE || BC       (Two lines are parallel if the corresponding angles formed are equal)

According to basic proportionality theorem if a line is parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

In ∆ABC, DE || BC

Hence we got the result .

#### Question 49:

In the given figure, RS || DB || PQ. If CP = PD = 11 cm and DR = RA = 3 cm. Then the values of x and y are respectively.

(a) 12, 10
(b) 14, 6
(c) 10, 7
(d) 16, 8

#### Answer:

Given: RS || DB || PQ. CP = PD = 11cm and DR = RA = 3cm

To find: the value of x and y respectively.

This relation is satisfied by option (d).

Hence, x = 16 cm and y = 8cm

Hence the result is .

#### Question 50:

In the given figure, if PB || CF and DP || EF, then $\frac{\mathrm{AD}}{\mathrm{DE}}=$

(a) $\frac{3}{4}$
(b) $\frac{1}{3}$
(c) $\frac{1}{4}$
(d) $\frac{2}{3}$

#### Answer:

Given: PB||CF and DP||EF. AB = 2 cm and AC = 8 cm.

To find: AD: DE

According to BASIC PROPORTIONALITY THEOREM, if a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

In ∆ACF, PB || CF.

.....(1)

Again, DP||EF.

Hence we got the result .

#### Question 1:

In a ∆ABC, D and E are points on the sides AB and AC respectively such that DE || BC.

(i) If AD = 6 cm, DB = 9 cm and AE = 8 cm, find AC.
(ii) If $\frac{\mathrm{AD}}{\mathrm{DB}}=\frac{3}{4}$ and AC = 15 cm, find AE.
(iii) If $\frac{\mathrm{AD}}{\mathrm{DB}}=\frac{2}{3}$ and AC = 18 cm, find AE.
(iv) If AD = 4, AE = 8, DB = x − 4, and EC = 3x − 19, find x.
(v) If AD = 8 cm, AB = 12 cm and AE = 12 cm, find CE.
(vi) if AD = 4 cm, DB = 4.5 cm and AE = 8 cm, find AC.
(vii) If AD = 2 cm, AB = 6 cm, and AC = 9 cm, find AE.
(viii) If $\frac{\mathrm{AD}}{\mathrm{BD}}=\frac{4}{5}$ and EC = 2.5 cm, find AE.
(ix) If AD = x, DB = x − 2, AE = x + 2 and EC = x − 1, find the value of x.
(x) If AD = 8x − 7, DB = 5x − 3, AE = 4x − 3 and EC = (3x − 1), find the value of x.
(xi) If AD = 4x − 3, AE = 8x − 7, BD = 3x − 1 and CE = 5x − 3. find the volume x.
(xii) If AD = 2.5 cm, BD = 3.0 cm and AE = 3.75 cm find the length of AC.

#### Answer:

(i) It is given that and  DE || BC

We have to find the

Since

So (by Thales theorem)

Then

Hence

(ii)It is given that and

We have to find

Let

So (by Thales theorem)

Then

Hence

(iii)It is given that and

We have to find

Let and

So (by Thales theorem)

Then

Hence

(iv)It is given that ,, and .

We have to find

So (by Thales theorem)

Then

Hence

(v) It is given that , and .

We have to find CE.

So      (by Thales theorem)

Then

Hence

(vi) It is given that , and .

We have to find .

So (by Thales theorem)

Then

Hence

(vii) It is given that , and .

We have to find .

Now

DB = 6 − 2 = 4 cm

So (by Thales theorem)

Then     (Let)

Hence

(viii) It is given that and EC = 2.5 cm

We have to find .

So    (by Thales theorem)

Then

$AE=\frac{4×2.5}{5}=2$ cm

Hence

(ix) It is given that ,, and .

We have to find the value of .

So      (by Thales theorem)

Then

Hence

(x) It is given that AD = 8x − 7, DB = 5x − 3, AE = 4x − 3 and EC = 3x − 1.

We have to find the value of .

So   (by Thales theorem)

Then,

Hence,

(xi) It is given that AD = 4x − 3, BD = 3x − 1, AE = 8x − 7 and EC = 5x − 3.

We have to find the value of .

So      (by Thales theorem)

Then

$\left(4x-3\right)\left(5x-3\right)=\left(3x-1\right)\left(8x-7\right)$

Then

Hence

(xii) It is given that , and .

So       (by Thales theorem)

Then

Now

#### Question 2:

In a ∆ABC, D and E are points on the sides AB and AC respectively. For each of the following cases show that DE || BC :

(i) AB = 12 cm, AD = 8 cm, AE = 12 cm and AC = 18 cm.
(ii) AB = 5.6 cm, AD = 1.4 cm, AC = 7.2  and AE = 1.8 cm.
(iii) AB = 10.8 cm, BD = 4.5 cm, AC = 4.8 cm and AE = 2.8 cm.
(iv) AD = 5.7 cm, BD = 9.5 cm, AE = 3.3 cm and EC = 5.5 cm

#### Answer:

(i) It is given that and are point on sides AB and AC.

We have to prove that DE || BC.

According to Thales theorem we have

(Proportional)

Hence, DE || BC.

(ii) It is given that and are point on sides AB and AC.

We have to prove that DE || BC.

According to Thales theorem we have

(Proportional)

Hence, DE || BC.

(iii) It is given that and are point on sides AB and AC.

We have to prove that DE || BC.

According to Thales theorem we have

So

$AD=AB-DB=10.8-4.5=6.3$

And

$EC=AC-AE=4.8-2.8=2$

Now

Hence, DE || BC.

(iv) It is given that and are point on sides AB and AC.

We have to prove that DE || BC.

According to Thales theorem we have

(Proportional)

Hence, DE || BC.

#### Question 3:

In a ∆ABC, P and Q are points on sides AB and AC respectively, such that PQ || BC. If AP = 2.4 cm, AQ = 2 cm, QC = 3 cm and BC = 6 cm, find the AB and PQ.

#### Answer:

It is given that ,, and .

We have to find AB and .

So (by Thales theorem)

Then

Now

Since PQ$\parallel$BC, AB is a transversal, then
∠APQ = ∠ABC  (corresponding angles)

Since PQ$\parallel$BC, AC is a transversal, then
∠AQP = ∠ACB  (corresponding angles)

In ∆APQ and ∆ABC,

∠APQ = ∠ABC   (proved above)

∠AQP = ∠ACB   (proved above)

so, ∆ APQ ∼ ∆ ABC  (Angle Angle Similarity)

Since the corresponding sides of similar triangles are proportional, then

#### Question 4:

In a ∆ABC, D and E are points on AB and AC respectively such that DE || BC. If AD = 2.4 cm, AE = 3.2 cm, DE = 2 cm and BC = 5 cm, find BD and CE.

#### Answer:

It is given that ,, and .

We have to find BD and CE.

Since DE$\parallel$BC, AB is transversal, then

∠ADE = ∠ABC   (corresponding angles)

Since DE$\parallel$BC, AC is a transversal, then

∠AED = ∠ACB   (corresponding angles)

In ∆ADE and ∆ABC,

∠ADE = ∠ABC  (proved above)

∠AED = ∠ACB  (proved above)

so, ∆ADE ∼ ∆ABC (Angle Angle similarity)

Since, the corresponding sides of similar triangles are proportional, then

Hence, BD = 3.6 cm  and  CE = 4.8 cm.

#### Question 5:

In Fig. 4.35, state if PQ || EF.

#### Answer:

It is given that EP = 3 cm, PG = 3.9 cm, FQ = 3.6 cm and FQ = 2.4 cm.

We have to check that or not.

According to Thales theorem we have

$\frac{PG}{GE}=\frac{GQ}{FQ}$

Now,

$\frac{3.9}{3}\ne \frac{3.6}{2.4}$

Hence, it is not proportional.

So, PQ ∦ EF.

#### Question 6:

M and N are points on the sides PQ and PR respectively of a ∆PQR. For each of the following cases, state whether MN || QR.

(i) PM = 4 cm, QM = 4.5 cm, PN = 4 cm, NR = 4.5 cm
(ii) PQ = 1.28 cm, PR = 2.56 cm, PM = 0.16 cm, PN = 0.32 cm

#### Answer:

(1)It is given that , , and .

We have to check that or not.

According to Thales theorem we have

(Proportional)

Hence,

(2) It is given that PQ = 1.28 cm, PR = 2.56 cm, PM = 0.16 cm and PN = 0.32 cm.

We have to check that or not.

According to Thales theorem we have

Now,

$\frac{PM}{MQ}=\frac{0.16}{1.12}=\frac{1}{7}\phantom{\rule{0ex}{0ex}}\frac{PN}{NR}=\frac{0.32}{2.24}=\frac{1}{7}\phantom{\rule{0ex}{0ex}}\therefore \frac{0.16}{1.12}=\frac{0.32}{2.24}$

Hence,

#### Question 1:

Fill in the blanks using the correct word given in brackets :

(i) All circles are ......... (congruent, similar).
(ii) All squares are ........(similar, congruent).
(iii) All .......... triangles are similar (isosceles, equilateral):
(iv) Two triangles are similar, if their corresponding angles are .......... (proportional, equal)
(v) Two triangles are similar, if their corresponding sides are ........... (proportional, equal)
(vi) Two polygons of the same number of sides are similar, if (a) their corresponding angles are and (b) their corresponding sides are ...........(equal, proportional).

#### Answer:

(i) Since all circles have centre and circumference, therefore all circles are similar.

Hence

(ii) Since all squares have each angle and sides are proportional, therefore all squares are similar.

Hence

(iii) In equilateral triangle each angle is therefore all equilateral triangles are similar.

Hence

(iv) Two triangles are similar, if their corresponding angles are .

(v) Two triangles are similar, if their corresponding sides are.

(vi) Two polygons of same number of sides are similar, if (a) their corresponding angles are and (b) their corresponding sides are .

#### Question 7:

In three line segments OA, OB and OC points L, M, N respectively are so chosen that LM || AB and MN || BC but neither of L, M, N nor of A, B, C are collinear. Show that LN || AC.

#### Question 8:

If D and E are points on sides AB and AC respectively of a ∆ABC such that DE || BC and BD = CE. Prove that ∆ABC is isosceles.

#### Answer:

It is given that in , DE || BC and BD = CE.

We have to prove that ∆ABC is isosceles.

By Thales theorem we have

Now and

So

Hence

So, ∆ABC is isosceles

#### Question 2:

Write the truth value (T/F) of each of the following statements:

(i) Any two similar figures are congruent.
(ii) Any two congruent figures are similar.
(iii) Two polygons are similar, if their corresponding sides are proportional.
(iv) Two polygons are similar, if their corresponding angles are proportional.
(v) Two triangles are similar if their corresponding sides are proportional.
(vi) Two triangles are similar if their corresponding angles are proportional.

(i)

(ii)

(iii)

(iv)

(v)

(vi)

#### Question 1:

If a ∆ ABC, AD is the bisector of ∠A, meeting side BC at D.

(i) If BD = 2.5 cm, AB = 5 cm and AC = 4.2 cm, find DC.
(ii) If BD = 2 cm, AB = 5 cm and DC = 3 cm, find AC.
(iii) If AB = 3.5 cm, AC = 4.2 cm and DC = 2.8 cm, find BD.
(iv) If AB = 10 cm, AC = 14 cm and BC = 6 cm, find BD and DC.
(v) If AC = 4.2 cm, DC = 6 cm and BC = 10 cm. find AB.
(vi) If AB = 5.6 cm, AC = 6 cm and DC = 3 cm, find BC.
(vii) If AD = 5.6 cm, BC = 6 cm and BD = 3.2 cm, find AC.
(viii) If AB = 10 cm, AC = 6 cm and BC = 12 cm, find BD and DC.

#### Answer:

(i) It is given that , and .

In , is the bisector of, meeting side at .

We have to find .

Since is bisector

Then

Hence

(ii) It is given that , and.

In , is the bisector of, meeting side at .

We have to find .

Since is bisector

So (is bisector of and side)

Then

Hence

(iii) It is given that , and .

In , is the bisector of, meeting side at .

We have to find BD.

Since is bisector

So (is bisector of and side)

Then

Hence

(iv) It is given that , and .

In , is the bisector of, meeting side at .

We have to find BD and .

Since is bisector

So (is bisector of and side)

Let BD = x cm. Then CD = (6 − x) cm

Then,

$\frac{10}{14}=\frac{x}{6-x}\phantom{\rule{0ex}{0ex}}⇒14x=60-10x\phantom{\rule{0ex}{0ex}}⇒24x=60\phantom{\rule{0ex}{0ex}}⇒x=\frac{60}{24}=2.5$

Hence, BD = 2.5 cm and DC = 6 − 2.5 = 3.5 cm.

(v) It is given that , and .

In , is the bisector of, meeting side at .

We have to find .

Since is bisector

So

Then

Hence

(vi) It is given that , and .

In , is the bisector of, meeting side at .

We have to find .

Since is bisector

So

Then

So

Hence

(vii) If it is given that AB = 5.6 cm, and .

In, is the bisector of, meeting side at

(viii) It is given that , and .

In , is the bisector of, meeting side at .

We have to find BD and .

Since is bisector

So

Let BD = x cm

Then

Now

Hence and

#### Question 2:

In Fig. 4.57, AE is the bisector of the exterior ∠CAD meeting BC produced in E. If AB = 10 cm, AC = 6 cm and BC = 12 cm, find CE.

#### Answer:

It is given that is the bisector of the exterior

Meeting produced and, and

Since is the bisector of the exterior

So

Hence

#### Question 3:

In Fig. 4.58, ∆ABC is a triangle such that . Find the ∠BAD.

#### Answer:

It is given that in , , and .

We have to find .

In ,

Since , therefore, AD is the bisector of $\angle A$.

Hence,

#### Question 4:

In ∆ABC (Fig. 4.59), if ∠1 = ∠2, prove that $\frac{\mathrm{AB}}{\mathrm{AC}}=\frac{\mathrm{BD}}{\mathrm{DC}}$.

#### Answer:

We have to prove that .
In ∆ABC,

(Given)

So, AD is the bisector of .

Therefore,

#### Question 5:

D, E and F are the points on sides BC, CA and AB respectively of ∆ABC such that AD bisects ∠A, BE bisects ∠B and CF bisects ∠C. If AB = 5 cm, BC = 8 cm and CA = 4 cm, determine AF, CE and BD.

#### Answer:

It is given that , and .

We have to find , and BD.

Sinceis bisector of

So

Then,

$\frac{5}{4}=\frac{BD}{BC-BD}\phantom{\rule{0ex}{0ex}}⇒\frac{5}{4}=\frac{BD}{8-BD}\phantom{\rule{0ex}{0ex}}⇒40-5BD=4BD\phantom{\rule{0ex}{0ex}}⇒9BD=40$

So,

Since is the bisector of .

So,

$\frac{AB}{BC}=\frac{AE}{EC}\phantom{\rule{0ex}{0ex}}⇒\frac{AB}{BC}=\frac{AC-EC}{EC}$

So

Now since is the bisector of

So

So

Hence

And

#### Question 6:

In Fig. 4.60, check whether AD is the bisector of ∠A of ∆ABC in each of the following:

(i) AB = 5 cm, AC = 10 cm, BD = 1.5 cm and CD = 3.5 cm
(ii) AB = 4 cm, AC = 6 cm, BD = 1.6 cm and CD = 2.4 cm
(iii) AB = 8 cm, AC = 24 cm, BD = 6 cm and BC = 24 cm
(iv) AB = 6 cm, AC = 8 cm, BD = 1.5 cm and CD = 2 cm
(v) AB =  5 cm, AC = 12 cm, BD = 2.5 cm and BC = 9 cm

#### Answer:

(i) It is given that,, and .

We have to check whether is bisector of .

First we will check proportional ratio between sides.

Now

$\frac{AB}{AC}=\frac{5}{10}=\frac{1}{2}$

$\frac{BD}{CD}=\frac{1.5}{3.5}=\frac{3}{7}$

Since $\frac{AB}{AC}\ne \frac{BD}{CD}$

Hence is not the bisector of .

(ii) It is given that,,, and .

We have to check whether is bisector of .

First we will check proportional ratio between sides.

So
$⇒\frac{4}{6}=\frac{1.6}{2.4}\phantom{\rule{0ex}{0ex}}⇒\frac{2}{3}=\frac{2}{3}\phantom{\rule{0ex}{0ex}}$

(It is proportional)

Hence, is bisector of .

(iii) It is given that,,, and .

We have to check whether is bisector of .

First we will check proportional ratio between sides.

Now

So

(It is proportional)

Hence, is bisector of .

(iv) It is given that, AB = 6 cm, AC = 8 cm, BD = 1.5 cm and CD = 2 cm.

We have to check whether is bisector of .

First we will check proportional ratio between sides.

So

$⇒\frac{6}{8}=\frac{1.5}{2}\phantom{\rule{0ex}{0ex}}⇒\frac{3}{4}=\frac{3}{4}$

(It is proportional)

Hence is bisector of .

(v) It is given that AB = 5 cm, AC = 12 cm, BD = 2.5 cm and BC = 9 cm

We have to check whether is bisector of .

First we will check proportional ratio between sides.

Now

$\frac{AB}{AC}=\frac{5}{12}$

$\frac{BD}{CD}=\frac{2.5}{9}=\frac{5}{18}$

Since $\frac{AB}{AC}\ne \frac{BD}{CD}$

Hence is not the bisector of .

#### Question 7:

In Fig. 4.60, AD bisects ∠A, AB = 12 cm, AC = 20 cm and BD = 5 cm, determine CD.

#### Answer:

It is given that bisect . Also, , and .

We have to find .

Since is the bisector of

Then

Hence

#### Question 1:

(i) In the given figure, if AB || CD, find the value of x.

(ii) In the given figure, If AB || CD, find the value of x.

(iii) In the given figure, AB || CD, If OA = 3x − 19, OB = x − 4, OC = x − 3 and OD = 4, find x.

#### Answer:

(i) It is given that .

We have to find the value of .

Diagonals of the para

Now

So

Therefore and

Hence

(ii) It is given that

We have to find the value of

Now

So

Therefore or

Hence x = $\frac{1}{2}$ or x = 2.

For x = $\frac{1}{2}$, OD is negative.

Hence

(iii) It is given that.

And, and

We have to find the value of

Now

So

Therefore and

Hence or .

#### Question 1:

In the given figure, ∆ACB ∼ ∆APQ. If BC = 8 cm, PQ = 4 cm, BA = 6.5 cm and AP = 2.8 cm, find CA and AQ.

#### Answer:

It is given that .

,, and.

We have to find and .

Since

So

Similarly

Hence, and

#### Question 2:

A vertical stick 10 cm long casts a shadow 8 cm long. At the same time a tower casts a shadow 30 m long. Determine the height of the tower.

#### Answer:

We have to find the height of .

Now,

∆ABC $~$∆PQR      (AA Similarity)

Hence

#### Question 3:

In the given figure, AB || QR. Find the length of PB.

#### Answer:

It is given that

,and PR = 6 cm

We have to find .

Since

So

Hence,

#### Question 4:

In the given figure, XY || BC. Find the length of XY.

#### Answer:

It is given that .

,and

We have to find .

Since

()

So

Hence,

#### Question 5:

In a right angled triangle with sides a and b and hypotenuse c, the altitude drawn on the hypotenuse is x. Prove that ab = cx.

#### Answer:

Let ∆ABC be a right angle triangle having sides and ; and hypotenuse . BD is the altitude drawn on the hypotenuse AC.

We have to find to prove .

Since the altitude is perpendicular on the hypotenuse, both the triangles are similar

Hence, .

#### Question 6:

In the given figure, ∠ABC = 90° and BD ⊥ AC. If BD = 8 cm and AD = 4 cm, find CD.

#### Answer:

It is given that and

When ,we have to find the.

Since is right angle triangle and is perpendicular on , so

(AA similarity)

Hence,

#### Question 7:

In the given figure, ∠ABC = 90° and BD ⊥ AC. If AB = 5.7 cm, BD = 3.8 cm and CD = 5.4 cm, find BC.

#### Answer:

It is given that,,, and

We have to find .

Since

So

Hence,

#### Question 8:

In the given figure, DE || BC such that AE = (1/4) AC. If AB = 6 cm, find AD.

#### Answer:

It is given that, and .

We have to find AD.

Since

So

Hence,

#### Question 9:

In the given figure, PA, QB and RC are each perpendicular to AC. Prove that $\frac{1}{x}+\frac{1}{z}=\frac{1}{y}$.

#### Answer:

It is given that , and are each perpendicular to .

We have to prove that

In we have

$⇒\frac{y}{x}=\frac{CB}{CA}$  ......(1)

Now in , we have

…… (2)

Adding (1) and (2) we have

Hence, .

#### Question 14:

D is the mid-point of side BC of a ∆ABC. AD is bisected at the point E and BE produced cuts AC at the point X. Prove that BE = EX = 3 : 1

#### Answer:

Given: ABC is a triangle in which D is the mid point of BC, E is the mid point of AD. BE produced meets AC at X.
To Prove: BE : EX = 3:1.
Construction: We draw a line DY parallel to BX.

Proof:

#### Question 16:

In ∆ABC, AL and CM are the perpendiculars from the vertices A and C to BC and AB respectively. If AL and CM intersect at O, prove that:

(i) ∆OMA ∼ ∆OLC
(ii) $\frac{\mathrm{OA}}{\mathrm{OC}}=\frac{\mathrm{OM}}{\mathrm{OL}}$

#### Question 10:

In the given figure, ∠A = ∠CED, prove that ∆CAB ∼ ∆CED. Also, find the value of x.

#### Answer:

Comparing ΔCAB and ΔCED,

CAB = CED                [Given]

ACB = ECD                [Common]

∴ ΔCAB ΔCED

#### Question 11:

The perimeters of two similar triangles are 25 cm and 15 cm respectively. If one side of first triangle is 9 cm, what is the corresponding side of the other triangle?

#### Answer:

It is given that perimeter of two similar triangle are and and one side.

We have to find the other side.

Let the corresponding side of the other triangle be x cm.

Hence

#### Question 12:

In ∆ABC and ∆DEF, it is being given that: AB = 5 cm, BC = 4 cm and CA = 4.2 cm; DE = 10 cm, EF = 8 cm and FD = 8.4 cm. If AL ⊥ BC and DM ⊥ EF, find AL : DM.

#### Answer:

It is given that ,,,, and .

We have to find

Since both triangle are similar

So,

Here, we use the result that in similar triangle the ratio of corresponding altitude is same as the ratio of the corresponding sides.

AL DM  = 1 : 2

#### Question 13:

D and E are the points on the sides AB and AC respectively of a ∆ABC such that: AD = 8 cm, DB = 12 cm, AE = 6 cm and CE = 9 cm, Prove that BC = 5/2 DE.

#### Answer:

It is given that,, and .

We have to prove that

Since clearly

Also, is common in and

So     (SAS Similarity)

#### Question 15:

ABCD is a parallelogram and APQ is a straight line meeting BC at P and DC produced at Q. Prove that the rectangle obtained by BP and DQ is equal to the rectangle contained by AB and BC.

#### Answer:

Given:
ABCD is a parallelogram and APQ is a straight line meeting BC at P and DC produced at Q.

To Prove:
The rectangle obtained by BP and DQ is equal to the rectangle contained by AB and BC. We need to prove that $\mathrm{BP}×\mathrm{DQ}=\mathrm{AB}×\mathrm{BC}$

Proof:
In ∆ABP and ​ ∆QCP, we have
∠ABP = ​ ∠QCP                      (Alternate angles as AB || DC)
∠BPA = ​ ∠QPC                      ( Vertically opposite angles)
By AA similarity, we get
∆ABP ~ ∆QCP
We know that corresponding sides of similar triangles are proportional.

$⇒\frac{\mathrm{AB}}{\mathrm{QC}}=\frac{\mathrm{BP}}{\mathrm{CP}}=\frac{\mathrm{AP}}{\mathrm{QP}}$

$⇒\frac{\mathrm{AB}}{\mathrm{QC}}=\frac{\mathrm{BP}}{\mathrm{CP}}\phantom{\rule{0ex}{0ex}}⇒\mathrm{AB}×\mathrm{CP}=\mathrm{QC}×\mathrm{BP}$

Adding $\mathrm{AB}×\mathrm{BP}$ in both sides, we get

#### Question 17:

In the given figure, we have AB || CD || EF. If AB = 6 cm, CD = x cm, EF = 10 cm, BD = 4 cm and DE = y cm, Calculate the values of x and y.

#### Answer:

It is given that .

,and

We have to calculate the values of and .

In and , we have

(Vertically opposite angles)

(Alternate interior angles)

So
$\frac{EF}{AB}=\frac{DE}{DB}$

Similarly in we have

Hence, $x=3.75\mathrm{cm}$ and

#### Question 18:

ABCD is a quadrilateral in which AD = BC. If P, Q, R, S be the mid-points of AB, AC, CD and BD respectively, show that PQRS is a rhombus.

#### Answer:

Given:
ABCD is quadrilateral in which AD = BC and P, Q, R, S are the mid points of AB, AC, CD, BD respectively.

To Prove:
PQRS is  a rhombus.

Proof:
In ∆ABC, P and Q are the midpoints of the sides AB and AC respectively.
By the Mid point theorem, we get
PQ || BC and PQ = $\frac{1}{2}$BC                                                                  ...(1)
In ∆ADC, Q and R are the midpoints of the sides AC and DC respectively.
By the Mid point theorem, we get
QR || AD and QR = $\frac{1}{2}$AD = $\frac{1}{2}$BC           (Since AD = BC)         ...(2)

Similarly, in  ∆BCD, we have
RS || BC and RS = $\frac{1}{2}$BC                                                                  ...(3)
In  ∆BAD, we have
PS || AD and PS = $\frac{1}{2}$AD  = $\frac{1}{2}$BC           (Since AD = BC)          ...(4)
From the equations (1), (2), (3), (4), we get
PQ = QR = RS = RS

Thus, PQRS is a rhombus.

#### Question 19:

In Fig. 4.145, if AB ⊥ BC, DC ⊥ BC and DE ⊥ AC, prove that ∆CED ∼ ∆ABC.

#### Answer:

It is given that , and .

We have to prove that .

Now,

,, so.

In ∆ABC and ∆CED,

(Given)

$\angle A=\angle ECD$           ( Alternate angles)

So,     (AA similarly rule)

#### Question 20:

In an isosceles ∆ABC, the base AB is produced both the ways to P and Q such that AP ✕ BQ = AC2. Prove that ∆APC ∼ ∆BCQ.

#### Answer:

It is given that is isosceles and .

We have to prove that .

It is given that is an isosceles triangle, so AC = BC.

Now,

(Given)

$⇒\frac{AP}{AC}=\frac{BC}{BQ}$
Also,

Hence,            (SAS Similarity)

#### Question 21:

A girl of heigh 90 cm is walking away from the base of a lamp-post at a speed of 1.2 m/sec. If the lamp is 3.6 m above the ground, find the length of her shadow after 4 seconds.

#### Answer:

It is given that, girl height, speed and height of lamp.

We have to find the length of her shadow after

Let be the lamp post and be the girl.

Suppose is the length of her shadow.
Let DE = x

And

Now in and we have

and

So by similarly criterion

$\frac{BE}{DE}=\frac{AB}{CD}\phantom{\rule{0ex}{0ex}}\frac{4.8+x}{x}=\frac{3.6}{0.9}=4\phantom{\rule{0ex}{0ex}}⇒3x=4.8\phantom{\rule{0ex}{0ex}}⇒x=1.6\phantom{\rule{0ex}{0ex}}$

Hence the length of her shadow after is 1.6 m.

#### Question 22:

Diagonals AC and BD of a trapezium ABCD with AB || DC intersect each other at the point O. Using similarity criterion for two triangles, show that $\frac{\mathrm{OA}}{\mathrm{OC}}=\frac{\mathrm{OB}}{\mathrm{OD}}$.

#### Answer:

It is given that trapezium with . O is the point of intersection of AC  and BD.

We have to prove that

Now, in and

$\angle AOB=\angle COD$    (Vertically opposite angles)

$\angle OAB=\angle OCD$    (Alternate angles)

$~$        (AA Similarity)

Hence,                 (Corresponding sides are proportional)

#### Question 23:

If ∆ABC and ∆AMP are two right triangles, right angled at B and M respectively such that ∠MAP = ∠BAC. Prove that

(i) ∆ABC ∼ ∆AMP
(ii) $\frac{\mathrm{CA}}{\mathrm{PA}}=\frac{\mathrm{BC}}{\mathrm{MP}}$

#### Answer:

(1) It is given that and are two right angle triangles.

Now, in and , we have

(Given)

(AA Similarity)

(2)

So,          (Corresponding sides are proportional)

#### Question 24:

A vertical stick of length 6 m casts a shadow 4 m long on the ground and at the same time a tower casts a shadow 28 m long. Find the height of the tower.

#### Answer:

It is given that length of vertical stick

We have to find the height of the tower.

Suppose is the height of the tower and is its shadow.

Now,

Hence the height of the tower is .

#### Question 25:

In the given figure, ∆ABC is right angled at C and DE ⊥ AB. Prove that ∆ABC ∼ ∆ADE and hence find the length of AE and DE.

#### Answer:

It is given that is right angle triangle and

We have to prove that and find the lengths of and .

In ∆ABC ∼ ∆ADE,

So by similarly criterion, we have

Since

So

And

Hence, and

#### Question 1:

Triangles ABC and DEF are similar.

(i) If area (∆ABC) = 16 cm2, area (∆DEF) = 25 cm2 and BC = 2.3 cm, find EF.
(ii) If area (∆ABC) = 9 cm2, area (∆DEF) = 64 cm2 and DE = 5.1 cm , find AB.
(iii) If AC = 19 cm and DF = 8 cm, find the ratio of the area of two triangles.
(iv) If area (∆ABC) = 36 cm2, area (∆DEF) = 64 cm2 and DE = 6.2 cm, find AB.
(v) If AB = 1.2 cm and DE = 1.4 cm, find the ratio of the areas of ∆ABC and ∆DEF.

#### Answer:

Given: ΔABC and ΔDEF are similar triangles

To find:

(i) If area of ΔABC = 16cm2, area of ΔDEF = 25cm2 and BC = 2.3 cm, Find EF.

(ii) If area of ΔABC = 9cm2, area of ΔDEF = 64cm2 and DE = 5.1 cm, Find AB.

(iii) If AC = 19cm and DF = 8cm, find the ratio of the area of two triangles.

(iv) If area of ΔABC = 36cm2, area of ΔDEF = 64cm2 and DE = 6.2 cm, Find AB.

(v) If AB = 1.2cm and DE = 1.4cm, find the ratio of the area of two triangles.

(i) We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

(ii)

(iii)

(iv)

(v)

#### Question 2:

In the given figure, ∆ACB ∼ ∆APQ. If BC = 10 cm, PQ = 5 cm, BA = 6.5 cm and AP = 2.8 cm, find CA and AQ. Also, find the area (∆ACB) : area (∆APQ).

#### Answer:

Given: ΔACB is similar to ΔAPQ.

BC = 10 cm, PQ = 5cm, BA = 6.5cm and AP = 2.8 cm

TO FIND:

(1) CA and AQ

(2) Area of ΔACB : Area of ΔAPQ

(1) It is given that ΔACB $~$ ΔAPQ.

We know that for any two similar triangles the sides are proportional. Hence

$\frac{\mathrm{AB}}{\mathrm{AQ}}=\frac{\mathrm{BC}}{\mathrm{PQ}}=\frac{\mathrm{AC}}{\mathrm{AP}}$

Similarly,

(2) We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

$\frac{\mathrm{ar}\left(∆\mathrm{ACQ}\right)}{\mathrm{ar}\left(∆\mathrm{APQ}\right)}={\left(\frac{\mathrm{BC}}{\mathrm{PQ}}\right)}^{2}={\left(\frac{10}{5}\right)}^{2}={\left(\frac{2}{1}\right)}^{2}=\frac{4}{1}$

#### Question 3:

The areas of two similar triangles are 81 cm2 and 49 cm2 respectively. Find the ratio of their corresponding heights. What is the ratio of their corresponding medians?

#### Answer:

Given: The area of two similar triangles is 81cm2 and 49cm2 respectively.

To find:

(1) Ratio of their corresponding heights.

(2) Ratio of their corresponding medians.

(1) We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding altitudes.

Taking square root on both sides, we get

(2) We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their medians.

Taking square root on both sides, we get

#### Question 4:

The areas of two similar triangles are 169 cm2 and 121 cm2 respectively. If the longest side of the larger triangle is 26 cm, find the longest side of the smaller triangle.

#### Answer:

Given: The area of two similar triangles is 169cm2 and 121cm2 respectively.The longest side of the larger triangle is 26cm.

To find: Longest side of the smaller triangle

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Taking square root on both sides, we get

= 22 cm

Hence, the longest side of the smaller triangle is .

#### Question 5:

Two isosceles triangles have equal vertical angles and their areas are in the ratio 36 : 25. Find the ratio of their corresponding heights.

#### Answer:

Given: Two isosceles triangles have equal vertical angles and their areas are in the ratio of 36:25.

To find: Ratio of their corresponding heights.

Suppose ∆ABC and ∆PQR are two isosceles triangles with $\angle \mathrm{A}=\angle \mathrm{P}$.

Now, AB = AC and PQ = PR

$\therefore \frac{\mathrm{AB}}{\mathrm{AC}}=\frac{\mathrm{PQ}}{\mathrm{PR}}$

In ∆ABC and ∆PQR,

$\angle \mathrm{A}=\angle \mathrm{P}$

$\frac{\mathrm{AB}}{\mathrm{AC}}=\frac{\mathrm{PQ}}{\mathrm{PR}}$

∴ ∆ABC $~$∆PQR     (SAS Similarity)

Let AD and PS be the altitudes of ∆ABC and ∆PQR, respectively.

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding altitudes.

$\therefore \frac{\mathrm{ar}\left(∆\mathrm{ABC}\right)}{\mathrm{ar}\left(∆\mathrm{PQR}\right)}={\left(\frac{\mathrm{AD}}{\mathrm{PS}}\right)}^{2}\phantom{\rule{0ex}{0ex}}⇒\frac{36}{25}={\left(\frac{\mathrm{AD}}{\mathrm{PS}}\right)}^{2}\phantom{\rule{0ex}{0ex}}⇒\frac{\mathrm{AD}}{\mathrm{PS}}=\frac{6}{5}$

Hence, the ratio of their corresponding heights is 6 : 5.

#### Question 6:

The areas of two similar triangles are  25 cm2 and 36 cm2 respectively. If the altitude of the first triangle is 2.4 cm, find the corresponding altitude of the other.

#### Answer:

Given: The area of two similar triangles is 25cm2 and 36cm2 respectively. If the altitude of first triangle is 2.4cm

To find: The altitude of the other triangle

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding altitudes.

Taking square root on both sides, we get

Hence, the corresponding altitude of the other is 2.88 cm.

#### Question 7:

The corresponding altitudes of two similar triangles are 6 cm and 9 cm respectively. Find the ratio of their areas.

#### Answer:

Given: The corresponding altitudes of two similar triangles are 6 cm and 9 cm respectively.

To find: Ratio of areas of triangle.

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding altitudes.

Hence, the ratio of areas of two triangles is 4 : 9.

#### Question 8:

ABC is a triangle in which ∠A = 90°, AN ⊥ BC, BC = 12 cm and AC = 5 cm. Find the ratio of the area of ∆ANC and ∆ABC.

#### Answer:

Given: In ΔABC,, , BC = 12cm and AC = 5cm.

TO FIND: Ratio of the triangles ∆ANC and ∆ABC.

In ∆ANC and ∆ABC,

$\angle ACN=\angle ACB$     (Common)

$\angle A=\angle ANC$          (90º each)

∴ ∆ANC $~$ABC     (AA Similarity)

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

#### Question 9:

In the given figure, DE || BC
(i) If DE = 4 cm, BC = 6 cm and Area (∆ADE) = 16 cm2, find the area of ∆ABC.
(ii) If DE = 4 cm, BC = 8 cm and Area (∆ADE) = 25 cm2, find the area of ∆ABC.
(iii) If DE : BC = 3 : 5. Calculate the ratio of the areas of ∆ADE and the trapezium BCED.

#### Answer:

In the given figure, we have DE || BC.

In ΔADE and ΔABC

So, $∆ADE~∆ABC$      (AA Similarity)

(i) We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence

(ii) We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence,

(iii) We know that

Let Area of ΔADE = 9x sq. units and Area of ΔABC = 25x sq. units

Now,

#### Question 10:

In ∆ABC, D and E are the mid-points of AB and AC respectively. Find the ratio of the areas of ∆ADE and ∆ABC.

#### Answer:

Given: In ΔABC, D and E are the midpoints of AB and AC respectively.

To find: Ratio of the areas of ΔADE and ΔABC.

Since it is given that D and E are the midpoints of AB and AC, respectively.

Therefore, DE || BC                   (Converse of mid-point theorem)

Also, $\mathrm{DE}=\frac{1}{2}\mathrm{BC}$

In ΔADE and ΔABC

So, $∆\mathrm{ADE}~∆\mathrm{ABC}$      (AA Similarity)

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

#### Question 11:

In the given figure, ∆ABC and ∆DBC are on the same base BC. If AD and BC intersect at O, prove that $\frac{\mathrm{Area}\left(∆\mathrm{ABC}\right)}{\mathrm{Area}\left(∆\mathrm{DBC}\right)}=\frac{\mathrm{AO}}{\mathrm{DO}}$.

#### Answer:

Given: ΔABC and ΔDBC are on the same base BC. AD and BC intersect at O.

Prove that:

Construction: Draw and .

Now, in ΔALO and ΔDMO, we have

$\therefore \frac{\mathrm{AL}}{\mathrm{DM}}=\frac{\mathrm{AO}}{\mathrm{DO}}$                (Corresponding sides are proportional)

#### Question 12:

ABCD is a trapezium in which AB || CD. The diagonals AC and BD intersect at O. Prove that : (i) ∆AOB ∼ ∆COD (ii) If OA = 6 cm, OC = 8 cm,

Find:
(a)
(b)

#### Answer:

Given: ABCD is a trapezium in which AB || CD.

The diagonals AC and BD intersect at O.

To prove:

(i)

(ii) If OA = 6 cm, OC = 8 cm

To find:

(a)

(b)

Construction: Draw a line MN passing through O and parallel to AB and CD

(i) Now in ΔAOB and ΔCOD

(ii) (a)We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

(b)We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

#### Question 13:

In ∆ABC, P divides the side AB such that AP : PB = 1 : 2. Q is a point in AC such that PQ || BC. Find the ratio of the areas of ∆APQ and trapezium BPQC.

#### Answer:

GIVEN: In ΔABC, P divides the side AB such that AP : PB = 1 : 2, Q is a point on AC such that PQ || BC.

TO FIND: The ratio of the areas of ΔAPQ and the trapezium BPQC.

In ΔAPQ and ΔABC

So, $∆\mathrm{APQ}~∆\mathrm{ABC}$      (AA Similarity)

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Let Area of ΔAPQ= 1 sq. units and Area of ΔABC = 9x sq. units

Now,

#### Question 14:

The areas of two similar triangles are 100 cm2 and 49 cm2 respectively. If the altitude of the bigger triangle is 5 cm, find the corresponding altitude of the other.

#### Answer:

Given: The area of two similar triangles is 100cm2 and 49cm2 respectively. If the altitude of bigger triangle is 5 cm

To find: their corresponding altitude of other triangle

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding altitudes.

Taking square root on both side

#### Question 15:

The areas of two similar triangles are 121 cm2 and 64 cm2 respectively. If the median of the first triangle is 12.1 cm, find the corresponding median of the other.

#### Answer:

Given: The area of two similar triangles is 121cm2 and 64cm2 respectively. IF the median of the first triangle is 12.1cm

To find: corresponding medians of the other triangle

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their medians.

#### Question 16:

If ∆ABC ∼ ∆DEF such that AB = 5 cm, area (∆ABC) = 20 cm2 and area (∆DEF) = 45 cm2, determine DE.

#### Answer:

Given: The area of two similar ΔABC = 20cm2, ΔDEF = 45cm2 respectively and AB = 5cm.

To find: measure of DE

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

#### Question 17:

In ∆ABC, PQ is a line segment intersecting AB at P and AC at Q such that PQ || BC and PQ divides ∆ABC into two parts equal in area. Find $\frac{\mathrm{BP}}{\mathrm{AB}}$.

#### Answer:

Given: In ΔABC, PQ is a line segment intersecting AB at P, and AC at Q such that PQ || BC and PQ divides ΔABC in two parts equal in area.

To find:

We have PQ || BC

And

Now, PQ || BC and BA is a transversal.

In ΔAPQ and ΔABC,

So, $∆\mathrm{APQ}~∆\mathrm{ABC}$      (AA Similarity)

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence

#### Question 18:

The areas of two similar triangles ABC and PQR are in the ratio 9 : 16. If BC = 4.5 cm, find the length of QR.

#### Answer:

Given: The areas of two similar triangles ABC and PQR are in the ratio 9 : 16. BC = 4.5cm.

To find: length of QR

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

$\frac{\mathrm{ar}\left(∆\mathrm{ABC}\right)}{\mathrm{ar}\left(∆\mathrm{PQR}\right)}={\left(\frac{\mathrm{BC}}{\mathrm{QR}}\right)}^{2}$

#### Question 19:

ABC is a triangle and PQ is a straight line meeting AB in P and AC in Q. If AP = 1 cm, PB = 3 cm, AQ = 1.5 cm, QC = 4.5 m, prove that the area of ∆APQ is one-sixteenth of the area of ∆ABC.

#### Answer:

Given: In ΔABC, PQ is a line segment intersecting AB at P, and AC at Q. AP = 1cm, PB = 3cm, AQ = 1.5cm and QC = 4.5cm.

To find:

In ΔABC,

According to converse of basic proportionality theorem if a line divides any two sides of a triangle in the same ratio, then the line must be parallel to the third side.

Hence, PQ || BC

In ΔAPQ and ΔABC,

So, $∆\mathrm{APQ}~∆\mathrm{ABC}$      (AA Similarity)

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Hence

#### Question 20:

If D is a point on the side AB of ∆ABC such that AD : DB = 3.2 and E is a point on BC such that DE || AC. Find the ratio of areas of ∆ABC and ∆BDE.

#### Answer:

Given: In ΔABC, D is a point on side AB such that AD : DB= 3 : 2. E is a point on side BC such that DE || AC.

To find:

In ΔABC,

According to converse of basic proportionality theorem if a line divides any two sides of a triangle in the same ratio, then the line must be parallel to the third side.

Hence DE || AC

In ΔBDE and ΔABC,

So, $∆\mathrm{BDE}~∆\mathrm{ABC}$      (AA Similarity)

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Let AD = 2x and BD = 3x.

Hence

#### Question 21:

If ∆ABC and ∆BDE are equilateral triangles, where D is the mid point of BC, find the ratio of areas of ∆ABC and ∆BDE.

#### Answer:

Given: In ΔABC and ΔBDE are equilateral triangles. D is the midpoint of BC.

To find: $\frac{\mathrm{Ar}\left(∆\mathrm{ABC}\right)}{\mathrm{Ar}\left(∆\mathrm{BDE}\right)}$

In ΔABC and ΔBDE

Since D is the midpoint of BC, BD : DC = 1.

We know that the ratio of areas of two similar triangles is equal to the ratio of squares of their corresponding sides.

Let DC = x, and BD = x

Therefore BC = BD + DC = 2x

Hence

#### Question 22:

AD is an altitude of an equilateral triangle ABC. On AD as base, another equilateral triangle ADE is constructed.
Prove that Area (∆ADE) : Area (∆ABC) = 3 : 4.

#### Answer:

We have an equilateral triangle
in which AD is altitude. An equilateral triangle
is drawn using AD as base. We have to prove that,

Since the two triangles are equilateral, the two triangles will be similar also.

We know that according to the theorem, the ratio of areas of two similar triangles is equal to the ratio of the squares of any two corresponding sides.

…… (1)

Nowis an equilateral triangle. So,

.

Therefore,

So,

We will now use this in equation (1). So,

Hence, proved.

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