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#### Page No 114:

#### Question 1:

Choose the correct answer from the given four options:

To divide a line segment AB in the ratio 5:7, first a ray AX is drawn so that ∠BAX is an acute angle and then at equal distances points are marked on the ray AX such that the minimum number of these points is

(A) 8

(B) 10

(C) 11

(D) 12

#### Answer:

Given, a line segment AB in the ratio 5 : 7.

A : B = 5 : 7

Now,

Draw a ray AX which makes an acute angle ∠BAX, then mark A + B points at equal distance. Here, A = 5 and B = 7.

Therefore, the minimum number of these points

= A + B

= 5 + 7

= 12.

Hence, the correct answer is option D.

#### Page No 114:

#### Question 2:

Choose the correct answer from the given four options:

To divide a line segment AB in the ratio 4:7, a ray AX is drawn first such that ∠BAX is an acute angle and then points A_{1}, A_{2}, A_{3}, .... are located at equal distances on the ray AX and the point B is joined to

(A) A_{12}

(B) A_{11}

(C) A_{10}

(D) A_{9}

#### Answer:

Given, a line segment AB in the ratio 4 : 7

$\Rightarrow $A : B = 4 : 7

Minimum number of points located at equal distances on the ray AX

= A + B

= 4 + 7

= 11

A_{1}, A_{2}, A_{3}, .......... are located at equal distances on the ray AX.

Point B is joined to the last point is A_{11}.

Hence, the correct answer is option B.

#### Page No 114:

#### Question 3:

Choose the correct answer from the given four options:

To divide a line segment AB in the ratio 5 : 6, draw a ray AX such that ∠BAX is an acute angle, then draw a ray BY parallel to AX and the points A_{1}, A_{2}, A_{3}, ... and B_{1}, B_{2}, B_{3}, ... are located at equal distances on ray AX and BY, respectively. Then the points joined are

(A) A_{5 }and B_{6}

(B) A_{6} and B_{5}

(C) A_{4} and B_{5}

(D) A_{5} and B_{4}

#### Answer:

Given, a line segment AB in the ratio 5 : 7

$\Rightarrow $A : B = 5 : 7

Steps of construction:

1. Draw a ray AX, an acute BAX.

2. Draw a ray BY$\parallel $AX,

$\therefore $$\angle $ABY = $\angle $BAX.

3. Now, locate the points A_{1}, A_{2}, A_{3}, A_{4} and A_{5} on AX and B_{1}, B_{2}, B_{3}, B_{4}, B_{5} and B_{6} on BY (Because A : B = 5 : 6)

4. Join A_{5}B_{6}.

$\therefore $ A_{5}B_{6} intersect AB at a point C.

$\Rightarrow $AC : BC = 5 : 6

Hence, the correct answer is option A.

#### Page No 114:

#### Question 4:

Choose the correct answer from the given four options:

To construct a triangle similar to a given ∆ABC with its sides $\frac{3}{7}$ of the corresponding sides of ∆ABC, first draw a ray BX such that ∠CBX is an acute angle and X lies on the opposite side of A with respect to BC. Then locate points B_{1}, B_{2}, B_{3}, ... on BX at equal distances and next step is to join

(A) B_{10 }to C

(B) B_{3 }to C

(C) B_{7} to C

(D) B_{4 }to C

#### Answer:

Steps:

1. Locate points B_{1}, B_{2}, B_{3}, B_{4}, B_{5}, B_{6} and B_{7} on BX at equal distance.

2. Join the last points is B_{7} to C.

Hence, the correct answer is option C.

#### Page No 114:

#### Question 5:

Choose the correct answer from the given four options:

To construct a triangle similar to a given ∆ABC with its sides $\frac{8}{5}$ of the corresponding sides of ∆ABC draw a ray BX such that ∠CBX is an acute angle and X is on the opposite side of A with respect to BC. The minimum number of points to be located at equal distances on ray BX is

(A) 5

(B) 8

(C) 13

(D) 3

#### Answer:

To construct a triangle similar to a triangle, with its sides $\frac{m}{n}$ of the corresponding sides of given triangle, the minimum number of points to be located at an equal distance is equal to the greater of m and n in $\frac{m}{n}$.

Here, $\frac{m}{n}=\frac{8}{5}$

So, the minimum number of point to be located at equal distance on ray BX is 8.

Hence, the correct answer is option C.

#### Page No 114:

#### Question 6:

Choose the correct answer from the given four options:

To draw a pair of tangents to a circle which are inclined to each other at an angle of 60°, it is required to draw tangents at end points of those two radii of the circle, the angle between them should be

(A) 135°

(B) 90°

(C) 60°

(D) 120°

#### Answer:

In the given figure,

OP and OQ are the tangents to the circle.

Now,

Sum of opposite angles = 180°

∴ $\angle $POQ + $\angle $PRQ = 180°

⇒ 60° + θ = 180°

⇒ θ = 120°

The angle between them should be 120°.

Hence, the correct answer is option D.

#### Page No 115:

#### Question 1:

Write True or False and give reasons for your answer

By geometrical construction, it is possible to divide a line segment in the ratio $\sqrt{3}:\frac{1}{\sqrt{3}}$.

#### Answer:

Given, ratio = $\sqrt{3}:\frac{1}{\sqrt{3}}$

$\therefore \mathrm{Required}\mathrm{ratio}=3:1[\mathrm{multiplying}\sqrt{3}\mathrm{in}\mathrm{each}\mathrm{term}]$

So, $\sqrt{3}:\frac{1}{\sqrt{3}}$ can be simplified as 3 : 1 and 3 as well as 1 both are positive integers.

Hence, by geometrical construction is possible to divide a line segment in the ratio 3 : 1.

#### Page No 115:

#### Question 2:

Write True or False and give reasons for your answer

To construct a triangle similar to a given ∆ABC with its sides $\frac{7}{3}$of the corresponding sides of ∆ABC, draw a ray BX making acute angle with BC and X lies on the opposite side of A with respect to BC. The points B_{1}, B_{2}, ...., B_{7} are located at equal distances on BX, B_{3} is joined to C and then a line segment B_{6}C*'* is drawn parallel to B_{3}C where C*'* lies on BC produced. Finally, line segment A*'*C*'* is drawn parallel to AC.

#### Answer:

False

Steps of construction

1. Draw a line segment BC.

2. B and C as centers draw two arcs of suitable radius intersecting each other at A.

3. Join BA and CA. ∆ABC is required triangle.

4. From B draw any ray BX downwards making an acute angle CBX.

5. Marked seven points B_{1}, B_{2}, B_{3 },............B_{7} on BX (BB_{1} = B_{1}B_{2} = B_{2}B_{3} = B_{3}B_{4} = B_{4}B_{5} = B_{5}B_{6} = B_{6}B_{7}).

6. Join B_{3}C and from B_{7} draw a line B_{7}C’ $\parallel $ B_{3}C intersecting the extended line segment BC at C’.

7. Draw C’A’ $\parallel $ CA intersecting the extended line segment BA at A’.

Then, ∆A’BC’ is the required triangle whose sides are $\frac{7}{3}$ of the corresponding sides of ∆ABC.

Given that, segment B_{6}C’ $\parallel $ B_{3}C. But since our construction is never possible that segment B_{6}C’ $\parallel $ B_{3}C because the similar triangle A’BC’ has its sides $\frac{7}{3}$ of the corresponding sides of triangle ABC.

So, B_{7}C’ is parallel to B_{3}C.

#### Page No 115:

#### Question 3:

Write True or False and give reasons for your answer

A pair of tangents can be constructed from a point P to a circle of radius 3.5 cm situated at a distance of 3 cm from the centre.

#### Answer:

False

Let, *r* = radius of circle and *d* = distance of a point from the center.

*r* = 3.5 and *d* = 3

$\because $ *r* > *d*

Point P lies in inside the circle, as shown below:

#### Page No 115:

#### Question 4:

Write True or False and give reasons for your answer

A pair of tangents can be constructed to a circle inclined at an angle of 170°.

#### Answer:

True

If the angle between the pair of tangents is always > 0° or <180 °,

Then we can construct a pair of tangents to a circle.

Hence, we can draw a pair of tangents to a circle inclined at an angle of 170°, as shown below:

#### Page No 116:

#### Question 1:

Draw a line segment of length 7 cm. Find a point P on it which divides it in the ratio 3 : 5.

#### Answer:

Steps of construction:

1. Draw a line segment AB = 7 cm.

2. Draw a ray AX, making an acute ∠BAX.

3. Along AX, mark 3 + 5 = 8 points A_{1}, A_{2}, A_{3}, A_{4}, A_{5}, A_{6}, A_{7}, A_{8} such that AA_{1} = A_{1}A_{2} = A_{2}A_{3} = A_{3}A_{4} = A_{4}A_{5} = A_{5}A_{6} = A_{6}A_{7} = A_{7}A_{8}.

4. Join A_{8}B.

5. From A_{3}, draw A_{3}P $\parallel $ A_{8}B meeting AB at P. (By making an angle equal to ∠BA_{8} at A_{3}).

Then P is the point on AB which divides it in the ratio 3 : 5. Thus, AP : PB = 3 : 5.

Justification:

Let

AA_{1} = A_{1}A_{2} = A_{2}A_{3} = A_{3}A_{4} =........= A_{7}A_{8} = *x*

In ∆ABA_{8}, A_{3}P $\parallel $ A_{8}B

$\therefore \frac{\mathrm{AP}}{\mathrm{PB}}=\frac{{\mathrm{AA}}_{3}}{{\mathrm{A}}_{3}{\mathrm{A}}_{8}}=\frac{3\mathrm{x}}{5\mathrm{x}}=\frac{3}{5}$

Hence, AP : PB = 3 : 5.

#### Page No 116:

#### Question 2:

Draw a right triangle ABC in which BC = 12 cm, AB = 5 cm and ∠B = 90°.

Construct a triangle similar to it and of scale factor $\frac{2}{3}$. Is the new triangle also a right triangle?

#### Answer:

Steps of construction:

1. Draw a line segment BC = 12 cm.

2. From B draw a line which makes a right angle.

a. Now as point B is the initial point as the centre, draw an arc any radius such that, the arc meets the ray BC at point D.

b. With D as centre and with the same radius as before, draw another arc cutting the previous one at point E.

c. Now with E as centre and with the same radius, draw an arc cutting the first arc (drawn in step a) at point F.

d. With E and F as centres, and with a radius more than half the length of FE, draw two arcs intersecting at point G.

e. Join points B and G. The angle formed by GBC is 90°. i.e. ∠ GBC = 90°.

3. From B as centre draw an arc of 5 cm which intersects the line GB at A.

4. Join AC, ABC is the given right triangle.

5. From B draw an acute $\angle $CBH downwards.

6. On ray BH, mark three point B_{1},B_{2} and B_{3}, such that BB_{1} = B_{1}B_{2} = B_{2}B_{3}.

7. Join B_{3}C

8. From point B_{2} draw B_{2}N || B_{3}C intersect BC at N.

9. From point N draw NM | | CA intersect BA at M. ∆MBN is the required triangle. ∆MNB is also a right angled triangle at B.

Justification:

As per the construction, ∆MNB is the required triangle

Let BB_{1} = B_{1}B_{2} = B_{2}B_{3} = *x*

Considering ∆BNB_{2} and ∆BCB_{3},

∠ B = ∠ B [same]

∠BNB_{2} =∠BCB_{3 } [corresponding angles of the same transverse as B_{3}C || B_{2}N]

$\therefore $$\u2206$BNB_{2} $~$$\u2206$BCB_{3 } [ by AA criteria ]

$\therefore \frac{\mathrm{BN}}{\mathrm{BC}}=\frac{{\mathrm{BB}}_{2}}{{\mathrm{BB}}_{3}}=\frac{2x}{3x}=\frac{2}{3}$ .....(1)

Similarly,

In ∆MBN and ∆ABC ,

∠ A = ∠ A [same]

∠BAC = ∠BMN [corresponding angles of the same transverse as AC || MN]

∆MBN $~$ ∆ABC [ by AA criteria ]

$\therefore \frac{\mathrm{MB}}{\mathrm{AB}}=\frac{\mathrm{MN}}{\mathrm{AC}}=\frac{\mathrm{BN}}{\mathrm{BC}}=\frac{2}{3}$ .....(2)

$\therefore $Constructed triangle ∆MNB is of scale $\frac{2}{3}$ times of the ∆ABC.

Also, as we can clearly see, ∠B = 90° is common in both the triangles ∆ABC and ∆MNB. Hence the constructed triangle ∆MNB is also a right angle triangle.

#### Page No 116:

#### Question 3:

Draw a triangle ABC in which BC = 6 cm, CA = 5 cm and AB = 4 cm. Construct a triangle similar to it and of scale factor $\frac{5}{3}$.

#### Answer:

Steps of construction:

1. Draw the line segment BC = 6 cm.

2. Taking B and C as centres, draw two arcs of radii 4 cm and 5 cm respectively intersecting each other at A.

3. Join BA and CA. ∆ABC is the required triangle.

4. From B, draw any ray BD downwards making at acute angle.

5. Mark five points B_{1}, B_{2}, B_{3}, B_{4} and B_{5} on BD, such that

BB_{1 }= B_{1}B_{2} = B_{2}B_{3} = B_{3}B_{4} = B_{4}B_{5}

6. Join B_{3}C and from B_{5} draw B_{5}M || B_{3}C intersecting the extended line segment BC at M.

7. From point M draw MN || CA intersecting the extended line segment BA at N.

8. Then, ∆NBM is the required triangle whose sides are equal to $\frac{5}{3}$ of the corresponding sides of the ∆ABC.

Justification:

As per the construction, ∆MNB is the required triangle

Let

BB_{1} = B_{1}B_{2} = B_{2}B_{3} = B_{3}B_{4} = B_{4}B_{5} = *x*

In triangles ∆BCB_{3} and ∆BMB_{5},

∠ B = ∠ B [same angle]

∠BCB_{3} = ∠BMB_{5} [corresponding angles of the same transverse as B_{5}D || B_{3}C]

$\therefore $ ∆BCB_{3} $~$ ∆BMB_{5} [by AA congruency criteria]

$\Rightarrow \frac{\mathrm{BM}}{\mathrm{BC}}=\frac{{\mathrm{BB}}_{5}}{{\mathrm{BB}}_{3}}=\frac{5x}{3x}=\frac{5}{3}$ .....(1)

Similarly, in ∆MBN and ∆CBA

∠ B = ∠ B [same angle]

∠BAC = ∠BNM [corresponding angles of the same transverse asAC || MN]

$\therefore $∆MBN $~$ ∆CBA [by AA congruency criteria]

$\Rightarrow \frac{\mathrm{AB}}{\mathrm{NB}}=\frac{\mathrm{AC}}{\mathrm{NM}}=\frac{\mathrm{BM}}{\mathrm{BC}}=\frac{5}{3}$ .....(2)

From (1) and (2) the constructed triangle ∆NBM is of scale $\frac{5}{3}$ times of the ∆ABC.

#### Page No 116:

#### Question 4:

Construct a tangent to a circle of radius 4 cm from a point which is at a distance of 6 cm from its centre.

#### Answer:

Steps of construction

1. Draw a circle of radius 4 cm. Let centre of this circle be O.

2. Take a point M at 6 cm away from the radius.

3. Join OM and bisect it. Now, with M and O as centres and with radius more than half of draw two arcs on the either sides of the line OM. Let the arc meet at A and B just that, M_{1} be mid-point of OM.

4. Taking M_{1} as centre and M_{1}O as radius draw a circle to intersect circle with radius 4 and centre O at two points P and Q.

5. Join PM and QM. PM and QM are the required tangents from M to circle with centre O and radius 4.

#### Page No 117:

#### Question 1:

Two line segments AB and AC include an angle of 60° where AB = 5 cm and AC = 7 cm. Locate points P and Q on AB and AC, respectively such that $\mathrm{AP}=\frac{3}{4}\mathrm{AB}\mathrm{and}\mathrm{AQ}=\frac{1}{4}\mathrm{AC}$. Join P and Q and measure the length PQ.

#### Answer:

Thinking process:

I. Firstly, we find the ratio of AB in which P divides it with the help of the relation AP = $\frac{3}{4}$ AB.

II. Secondly, we find the ratio of AC in which Q divides it with the help of the relation AQ = $\frac{1}{4}$ AC.

III. Now, construct the line segment AB and AC in which P and Q respectively divides it in the ratio from step

(i) and (ii) respectively.

IV. Finally get the points P and Q. After that join PQ and get the required measurement of PQ.

Given that, AB = 5 cm and AC = 7 cm

Also, $\mathrm{AP}=\frac{3}{4}\mathrm{AB}\mathrm{and}\mathrm{AQ}=\frac{1}{4}\mathrm{AC}$ .....(1)

$\Rightarrow \mathrm{AP}=\frac{3}{4}\times 5=\frac{15}{4}$ cm

Then, PB =AB $-$ AP

$\Rightarrow \mathrm{PB}=5-\frac{15}{4}=\frac{5}{4}\mathrm{cm}$

$\Rightarrow \mathrm{AP}:\mathrm{PB}=\frac{15}{4}:\frac{5}{4}$

Hence AP : PB = 3 : 1

i.e. scale factor of line segment AB is $\frac{3}{1}$.

Again from (1)

$\mathrm{AQ}=\frac{1}{4}\mathrm{AC}=\frac{1}{4}\times 7=\frac{7}{4}$ cm

Then,

$\mathrm{QC}=\mathrm{AC}-\mathrm{AQ}=7-\frac{7}{4}=\frac{21}{4}\mathrm{cm}$

$\therefore \mathrm{AQ}:\mathrm{QC}=\frac{7}{4}:\frac{21}{4}$

Hence AQ : QC = 1 : 3

i.e. scale factor of line segment AQ is $\frac{1}{3}$.

Steps of construction:

1. Draw a line segment AB = 5cm.

2. Now draw a ray AZ making an acute ∠BAZ=60°.

3. With A as centre and radius equal to 7 cm draw an arc intersecting the line AZ at C.

4. Draw a ray AX, making an acute ∠BAX.

5. Along AX, mark 1+3 = 4 points A_{1}, A_{2}, A_{3} and A_{4}

Such that A_{1}A_{2} = A_{1}A_{3} = A_{3}A_{4
}

6. Join A_{4}B

7. From A_{3} draw A_{3}P || A_{4}B meeting AB at P. [by making an angle equal to ∠AA_{4}B]

Then, P is the point on AB which divides it in the ratio 3 : 1.

So, AP : PB = 3 : 1

8. Draw a ray AY, making an acute ∠CAY.

9. Along AY, mark 3+1 = 4 point B_{1, }B_{2}, B_{3} and B_{4}.

Such that AB_{1} = B_{1}B_{2} = B_{2}B_{3} = B_{3}B_{4}

10. Join B_{4}C.

11. From B_{1} draw B_{1}Q || B_{4}C meeting AC at Q. [by making an angle equal to∠AB_{4}C]

Then, Q is the point on AC which divides it in the ratio 1 : 3.

So, AQ : QC = 1 : 3

12. Finally, join PQ and its measurement is 3.25 cm.

#### Page No 117:

#### Question 2:

Draw a parallelogram ABCD in which BC = 5 cm, AB = 3 cm and ∠ABC = 60°, divide it into triangles BCD and ABD by the diagonal BD.

Construct the triangle BD*'* C*'* similar to ∆BDC with scale factor $\frac{4}{3}$. Draw the line segment D*'*A*'* parallel to DA where A*'* lies on extended side BA. Is A*'*BC*'*D*' *a parallelogram?

#### Answer:

Thinking process:

I. Firstly we draw a line segment, then either of one end of the line segment with length 5 cm and making an angle 60° with this end. We know that is parallelogram both opposite sides are equal and parallel, then again draw a line with 5 cm making an angle with 60° from other end of line segment. Now, join both parallel line by a line segment whose measurement is 3 cm, we get a parallelogram. After that we draw a diagonal and get a triangle BOC.

II. Now, we construct ∆BD'C' similar to ∆BDC with scale factor $\frac{4}{3}$.

III. Now, draw the line segment D'A' parallel to DA.

IV. Finally, we get the required parallelogram A'BC'D'.

Steps of construction:

1. Draw a line segment AB = 3 cm.

2. Now, draw a ray BY making an acute ∠ABY = 60°.

3. With B as centre and radius equal to 5 cm draw an arc cut the point C on BY.

4. Again draw a ray AZ making an acute ∠ZAX_{1} = 60°.

5. With A as centre and radius equal to 5 cm draw an arc cut the point D on AZ.

6. Now, join CD and finally make a parallelogram ABCD.

7. Join BD, which is a diagonal of parallelogram ABCD and from B draw any ray BX downwards making an acute ∠CBX.

8. Locate 4 points B_{1}, B_{2}, B_{3}, B_{4} on BX, such that BB_{1} = B_{1}B_{2} = B_{2}B_{3} = B_{3}B_{4}.

9. Join B_{3}C and from B_{4} draw a line B_{4}C' || B_{3}C intersecting the extended line segment BC at C'.

11. From point C' draw C'D'$\parallel $CD intersecting the extended line segment BD at D'. Then, ∆D'BC' is the required triangle whose sides are $\frac{4}{3}$ of the corresponding sides of ∆DBC.

12. Now draw a line segment D'A' parallel to DA, where A' lies on the extended side BA i.e. a ray BX_{1}.

13. Finally, we observe that A'BC'D' is a parallelogram in which A'D' = 6.5 cm A'B = 4 cm and ∠A'BD' = 60° divide it into triangles BC'D' and A'BD' by the diagonal BD'.

Justification that A'BC'D' is a parallelogram:

As per the construction,

CD || C'D' and BC' || A'D' .....(1)

Also,

As per the given data, ∠ ABC = 60°.

Consider the parallelogram ABCD, the sum of complementary angles is 180°.

So ∠ ABC + ∠ BCD = 180°

∠ BCD = 180° $-$ ∠ ABC = 180° $-$ 60° = 120°.

Therefore ∠ BCD = 120°.

Now, as CD || C'D',

∠ BCD = ∠ BC'D' (as corresponding angles)

Hence ∠ BC'D' = 120° .....(2)

As BC || AD and AD || A'D', hence BC || A'D',

∠ABC = ∠D'A'X_{1} (corresponding angles)

So ∠D'A'X_{1} = ∠ABC = 60°

Now consider ∠D'A'X_{1} + ∠ D'A'B = 180° (linear angle)

60° + ∠ D’A’B = 180°

$\Rightarrow $∠ D’A’B = 180° $-$ 60° = 120° .....(3)

From (1), (2) and (3), we can say that A’BC'D' is a parallelogram. (As opposite sides and opposite angles are equal).

#### Page No 117:

#### Question 3:

Draw two concentric circles of radii 3 cm and 5 cm. Taking a point on outer circle construct the pair of tangents to the other. Measure the length of a tangent and verify it by actual calculation.

#### Answer:

Given, two concentric circles of radii 3 cm and 5 cm with centre O. We have to draw pair of tangents from point P on outer circle to the other.

Steps of construction:

1. Draw two concentric circles with centre O and radii 3cm and 5cm.

2. Taking any point P on outer circle. Join OP.

3. Bisect OP, let M’ be the mid-point of OP.

4. Taking M’ as centre and OM’ as radius draw a circle dotted which cuts the inner circle at A and B.

5. Join PA and PB. Thus, PA and PB are required tangents.

6. On measuring PA and PB, we find that PA = PB = 4 cm.

Actual calculation:

In the right angle ∆OAP,

∠PAO = 90°

According to Pythagoras theorem

(hypotenuse)^{2} = (base)^{2} + (perpendicular)^{2}

$\Rightarrow $PA^{2} = (5)^{2} $-$ (3)^{2} = 25 $-$ 9 = 16

$\Rightarrow $PA = 4 cm

Hence, the length of both tangents is 4 cm.

Therefore, PA = PB = 4 cm.

#### Page No 117:

#### Question 4:

Draw an isosceles triangle ABC in which AB = AC = 6 cm and BC = 5 cm.

Construct a triangle PQR similar to ∆ABC in which PQ = 8 cm. Also justify the construction.

#### Answer:

Steps of construction:

1. Draw a line segment BC = 5 cm.

2. Construct OQ the perpendicular bisector of line segment BC meeting BC at P'.

3. Taking B and C as centres draw two arcs of equal radius 6 cm intersecting each other at A.

4. Join BA and CA. So, ∆ABC is the required isosceles triangle and from B, draw any ray BX making an acute angle, ∠CBX.

5. Locate four points B_{1}, B_{2}, B_{3} and B_{4} on BX such that BB_{1} = B_{1}B_{2} = B_{2}B_{3} = B_{3}B_{4
}

6. Join B_{3}C and from B_{4} draw a line B_{4}R || B_{3}C intersecting the extended line segment BC at R.

7. From point R, draw RP || CA meeting the extended line BA at P.

Then, ∆PBR is the required triangles.

Justification:

Let BB_{1} = B_{1}B_{2} = B_{2}B_{3} = B_{3}B_{4} = *x*

As per the construction B_{4}R $\parallel $B_{3}C

$\frac{\mathrm{BC}}{\mathrm{CR}}=\frac{{\mathrm{BB}}_{3}}{{\mathrm{B}}_{3}{\mathrm{B}}_{4}}=\frac{3x}{x}=\frac{3}{1}$

$\therefore \frac{\mathrm{BC}}{\mathrm{CR}}=\frac{3}{1}$

$\Rightarrow \frac{\mathrm{BR}}{\mathrm{BC}}=\frac{\mathrm{BC}+\mathrm{CR}}{\mathrm{BC}}=\frac{\mathrm{BC}}{\mathrm{BC}}+\frac{\mathrm{CR}}{\mathrm{BC}}=1+\frac{1}{3}=\frac{4}{3}$

Also, from the construction RP || CA

In $\u2206\mathrm{PBR}\mathrm{and}\u2206\mathrm{ABC}$,

$\angle \mathrm{PBR}=\angle \mathrm{ABC}$

$\angle \mathrm{PRB}=\angle \mathrm{ACB}$ [Corresponding Angles]

$\Rightarrow \u2206PBR~\u2206ABC$ [ By AA criteria]

and $\frac{\mathrm{PB}}{\mathrm{AB}}=\frac{\mathrm{RP}}{\mathrm{CA}}=\frac{\mathrm{BR}}{\mathrm{BC}}=\frac{4}{3}$

Hence, the new triangle is similar to the given triangle whose sides are $\frac{4}{3}$ times of the corresponding sides of the isosceles $\u2206$ABC.

#### Page No 117:

#### Question 5:

Draw a triangle ABC in which AB = 5 cm, BC = 6 cm and ∠ABC = 60º. Construct a triangle similar to ∆ABC with scale factor $\frac{5}{7}$. Justify the construction.

#### Answer:

Steps of construction:

1. Draw a line segment AB = 5 cm.

#### Page No 118:

#### Question 6:

Draw a circle of radius 4 cm. Construct a pair of tangents to it, the angle between which is 60º. Also justify the construction. Measure the distance between the centre of the circle and the point of intersection of tangents.

#### Answer:

Step - I A circle with center O and radius 4cm is drawn.

Step - II OQ is joined when Q is any point on the circle.

Step - III ∠OQR = 30$\xb0$ is drawn and QR intersects the circle at R.

Step - IV Centering Q and R and with radius QR two arcs are drawn, they intersect at P.

Step - V PQ and PR are joined.

PQ and PR are the required tangents.

Justification

PQ = QR = RP (By construction)

$\mathrm{i}.\mathrm{e}.\angle \mathrm{PQR}=\angle \mathrm{QRP}=\angle \mathrm{RPQ}=60\xb0\phantom{\rule{0ex}{0ex}}\therefore \angle \mathrm{OQP}=\angle \mathrm{OQR}+\angle \mathrm{PQR}\phantom{\rule{0ex}{0ex}}=30\xb0+60\xb0\phantom{\rule{0ex}{0ex}}=90\xb0$

Thus, PQ is a tangent at Q.

Again in ∆OQR, we have

$\angle \mathrm{OQR}=\angle \mathrm{ORQ}=30\xb0$

$\therefore \angle \mathrm{ORP}=\angle \mathrm{ORQ}+\angle \mathrm{QRP}\phantom{\rule{0ex}{0ex}}=30\xb0+60\xb0\phantom{\rule{0ex}{0ex}}=90\xb0$

Thus, PR is a tangent to circle at R

Distance between center and intersection of tangent i.e., OP = 8cm

#### Page No 118:

#### Question 7:

Draw a triangle ABC in which AB = 4 cm, BC = 6 cm and AC = 9 cm. Construct a triangle similar to ∆ABC with scale factor $\frac{3}{2}$. Justify the construction. Are the two triangles congruent? Note that all the three angles and two sides of the two triangles are equal.

#### Answer:

Step 1: Constant a triangle ABC with AB = 4cm, BC = 6cm and AC = 9cm.

Step 2: Draw a ray AX making an acute angle with the base AC and mark 3 points A1, A2, A3 such that AA1 = A1A2 = A2A3

Step 3: Join A2C and draw a line A3C′ such that A2C is parallel to A3C′ where C′ lie on the produced AC

Step 4: Now draw another line parallel to BC at C′ such that it meet the produced AB at B′

Here, ΔAB′C′ is the required triangle similar to ΔABC with scale factor $\frac{3}{2}$.

Justification

A

_{3}C′ $\parallel $ A

_{2}C

$\frac{\mathrm{AC}}{\mathrm{CC}\text{'}}=\frac{2}{1}$

$\frac{\mathrm{AC}\text{'}}{\mathrm{AC}}=\frac{\mathrm{AC}+\mathrm{CC}\text{'}}{\mathrm{AC}}=1+\frac{\mathrm{CC}\text{'}}{\mathrm{AC}}\phantom{\rule{0ex}{0ex}}=1+\frac{1}{2}\phantom{\rule{0ex}{0ex}}=\frac{3}{2}$

Again BC $\parallel $ B'C'

$\therefore \frac{\mathrm{AB}\text{'}}{\mathrm{AB}}=\frac{\mathrm{BC}\text{'}}{\mathrm{BC}}=\frac{\mathrm{AC}\text{'}}{\mathrm{AC}}=\frac{3}{2}$

Hence, $\u2206AB\text{'}C\text{'}~\u2206ABC$

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