Math Ncert Exemplar Solutions for Class 10 Maths Chapter 11 Area Related To Circles are provided here with simple step-by-step explanations. These solutions for Area Related To Circles are extremely popular among Class 10 students for Maths Area Related To Circles Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Math Ncert Exemplar Book of Class 10 Maths Chapter 11 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Math Ncert Exemplar Solutions. All Math Ncert Exemplar Solutions for class Class 10 Maths are prepared by experts and are 100% accurate.

#### Page No 120:

#### Question 1:

Choose the correct answer from the given four options:

If the sum of the areas of two circles with radii* R*_{1} and *R*_{2} is equal to the area of a circle of radius *R*, then

(A) *R*_{1} +* R*_{2 }= *R*

(B) ${R}_{1}^{2}+{R}_{2}^{2}={R}^{2}$

(C) *R*_{1 }+ *R*_{2} < *R*

(D) ${R}_{1}^{2}+{R}_{2}^{2}<{R}^{2}$

#### Answer:

Given that,

Area of circle = Area of first circle + Area of second circle

∴ $\mathrm{\pi}$*R*^{2} = $\mathrm{\pi}$*R*_{1}^{2} + $\mathrm{\pi}$*R*_{2}^{2}

⇒ *R*^{2} = *R*_{1}^{2} + *R*_{2}^{2}

Hence, the correct answer is option B.

#### Page No 120:

#### Question 2:

Choose the correct answer from the given four options:

If the sum of the circumferences of two circles with radii * R*_{1} and *R*_{2} is equal to the circumference of a circle of radius *R*, then

(A) *R*_{1} +* R*_{2 }= *R*

(B) *R*_{1} +* R*_{2 }> *R*

(C) *R*_{1 }+ *R*_{2} < *R*

(D) Nothing definite can be said about the relation among *R*_{1}, *R*_{2} and *R*.

#### Answer:

We are given that,

Circumference of circle with radius *R* = Circumference of first circle with radius *R*_{1} + Circumference of second circle with radius *R*_{2}

⇒ 2$\mathrm{\pi}$*R* = 2$\mathrm{\pi}$*R*_{1} + 2$\mathrm{\pi}$*R*_{2}

⇒ *R* = *R*_{1}+ *R*_{2}

Hence, the correct answer is option A.

#### Page No 121:

#### Question 3:

Choose the correct answer from the given four options:

If the circumference of a circle and the perimeter of a square are equal, then

(A) Area of the circle = Area of the square

(B) Area of the circle > Area of the square

(C) Area of the circle < Area of the square

(D) Nothing definite can be said about the relation between the areas of the circle and square.

#### Answer:

Let *r* be the radius of the circle and *a* be the side of square.

Given that,

Circumference of a circle = Perimeter of square

⇒ 2π*r* = 4*a*

$\Rightarrow \frac{22}{7}r=2a$

⇒ 11*r* = 7*a*

$\Rightarrow r=\frac{7}{11}a$ .....(1)

Now, area of circle, *A*_{1} = $\mathrm{\pi}$*r*^{2} and area of square, *A*_{2} = *a*^{2}

From (1),

*A*_{1} = $\mathrm{\pi}$ × ${\left(\frac{7}{11}a\right)}^{2}$

= $\frac{22}{7}\times \left(\frac{49}{121}\right){a}^{2}$

= $\frac{14}{11}{a}^{2}$

∴ *A*_{1} = $\left(\frac{14}{11}\right)$ *A*_{2 }(∵ *A*_{2} = *a*^{2})

⇒ *A*_{1} > *A*_{2}

∴ Area of the circle > Area of the square.

Hence, the correct answer is option B.

#### Page No 121:

#### Question 4:

Choose the correct answer from the given four options:

Area of the largest triangle that can be inscribed in a semi-circle of radius *r* units is

(A) *r*^{2} sq. units

(B) $\frac{1}{2}{r}^{2}\mathrm{sq}.\mathrm{units}$

(C) 2*r*^{2} sq. units

(D) $\sqrt{2}{r}^{2}\mathrm{sq}.\mathrm{units}$

#### Answer:

A largest triangle that can be inscribed in a semi-circle of radius *r* units is the triangle having its base as the diameter of the semi-circle and the two other sides are taken by considering a point C on the circumference of the semi-circle and joining it by the end points of diameter A and B.

∴ ∠C = 90° (by the properties of circle)

So, $\u25b3$ABC is right angled triangle with base as diameter AB of the circle and height be CD.

Now, the height of the triangle = *r*

∴ Area of largest $\u25b3$ABC = $\left(\frac{1}{2}\right)$× Base × Height

= $\left(\frac{1}{2}\right)$ × AB × CD

= $\left(\frac{1}{2}\right)$ × 2*r* × *r
*=

*r*sq. units

^{2}Hence, the correct answer is option (A).

#### Page No 121:

#### Question 5:

Choose the correct answer from the given four options:

If the perimeter of a circle is equal to that of a square, then the ratio of their areas is

(A) 22 : 7

(B) 14 : 11

(C) 7 : 22

(D) 11: 14

#### Answer:

Let *r* be the radius of the circle and *a* be the side of the square.

Given that,

Perimeter of a circle = Perimeter of a square

⇒ 2$\mathrm{\pi}$*r* = 4*a*

⇒ *a* = $\frac{\mathrm{\pi r}}{2}$

Now, Area of the circle = *r*^{2} and Area of the square = *a*^{2}

Therefore,

Ratio of their areas = $\frac{\mathrm{Area}\mathrm{of}\mathrm{Circle}}{\mathrm{Area}\mathrm{of}\mathrm{Square}}$

= $\frac{\mathrm{\pi}{r}^{2}}{{a}^{2}}$

= $\frac{\mathrm{\pi}{r}^{2}}{{\left({\displaystyle \frac{\mathrm{\pi}r}{2}}\right)}^{2}}$

= $\frac{\mathrm{\pi}{r}^{2}}{\left({\displaystyle \frac{{\mathrm{\pi}}^{2}{r}^{2}}{4}}\right)}$

= $\frac{14}{11}$

Hence, the correct answer is option B.

#### Page No 121:

#### Question 6:

Choose the correct answer from the given four options:

It is proposed to build a single circular park equal in area to the sum of areas of two circular parks of diameters 16 m and 12 m in a locality. The radius of the new park would be

(A) 10 m

(B) 15 m

(C) 20 m

(D) 24 m

#### Answer:

Let *D*_{1} be the diameter of the first circular park = 16 m

∴ Radius of first circular park = 8 m

Let *D*_{2} be the diameter of the second circular park = 12 m

∴ Radius* *of second circular park = 6 m

Area of first circular park = $\mathrm{\pi}$*r ^{2}* = $\mathrm{\pi}$(8)

^{2}= 64$\mathrm{\pi}$ m

^{2}

Area of second circular park = $\mathrm{\pi}$

*r*= $\mathrm{\pi}$(6)

^{2}^{2}= 36$\mathrm{\pi}$ m

^{2}

Given that,

Area of single circular park = Area of first circular park + Area of second circular park

∴ $\mathrm{\pi}$

*R*

^{2}= 64$\mathrm{\pi}$ + 36$\mathrm{\pi}$

= 100π (where

*R*is the radius of the single circular park)

⇒ $\mathrm{\pi}$

*R*

^{2}= 100$\mathrm{\pi}$

⇒

*R*

^{2}= 100

⇒

*R*= 10

∴ Radius of the single circular park will be 10 m.

Hence, the correct answer is option A.

#### Page No 121:

#### Question 7:

Choose the correct answer from the given four options:

The area of the circle that can be inscribed in a square of side 6 cm is

(A) 36$\mathrm{\pi}$ cm^{2}

(B) 18$\mathrm{\pi}$ cm^{2}

(C) 12$\mathrm{\pi}$ cm^{2}

(D) 9$\mathrm{\pi}$ cm^{2}

#### Answer:

Side of square = 6 cm

∴ Diameter of a circle = Side of square = 6 cm

⇒ Radius of the circle = 3 cm

∴ Area of the circle = $\mathrm{\pi}$*r ^{2}*

= $\mathrm{\pi}$(3)

^{2}

= 9$\mathrm{\pi}$ cm

^{2}

Hence, the correct answer is option D.

#### Page No 121:

#### Question 8:

Choose the correct answer from the given four options:

The area of the square that can be inscribed in a circle of radius 8 cm is

(A) 256 cm^{2}

(B) 128 cm^{2}

(C) $64\sqrt{2}{\mathrm{cm}}^{2}$

(D) 64 cm^{2}

#### Answer:

Let the radius of circle, *r* = OC = 8 cm.

∴ Diameter of the circle = AC = 16 cm

Let *a* be the side of the square.

Given that,

Diagonal of square = Diameter of the circle.

Now in right angled triangle $\u25b3$ACB, by Pythagoras theorem,

(AC)^{2} = (AB)^{2} + (BC)^{2}

⇒ (16)^{2} = *a*^{2} + *a*^{2}

⇒ 256 = 2*a*^{2}

⇒ *a*^{2} = 128

∴ Area of the square = *a*^{2} = 128 cm^{2}

Hence, the correct answer is option (B).

#### Page No 121:

#### Question 9:

Choose the correct answer from the given four options:

The radius of a circle whose circumference is equal to the sum of the circumferences of the two circles of diameters 36 cm and 20 cm is

(A) 56 cm

(B) 42 cm

(C) 28 cm

(D) 16 cm

#### Answer:

Diameter of first circle, *d*_{1} = 36 cm

Diameter of second circle, *d*_{2} = 20 cm

∴ Circumference of first circle = $\mathrm{\pi}$*d*_{1} = 36$\mathrm{\pi}$ cm

Circumference of second circle = $\mathrm{\pi}$*d*_{2} = 20$\mathrm{\pi}$ cm

Now, let *D* be the diameter of the new circle formed.

Given that,

Circumference of circle = Circumference of first circle + Circumference of second circle

$\mathrm{\pi}$*D* = $\mathrm{\pi}$*d*_{1} + $\mathrm{\pi}$*d*_{2}

⇒ $\mathrm{\pi}$*D* = 36$\mathrm{\pi}$ + 20$\mathrm{\pi}$

⇒ $\mathrm{\pi}$*D* = 56$\mathrm{\pi}$

⇒ *D* = 56

⇒ Radius = $\frac{56}{2}$ = 28 cm

Hence, the correct answer is option (C).

#### Page No 121:

#### Question 10:

Choose the correct answer from the given four options:

The diameter of a circle whose area is equal to the sum of the areas of the two circles of radii 24 cm and 7 cm is

(A) 31 cm

(B) 25 cm

(C) 62 cm

(D) 50 cm

#### Answer:

Area of first circle = $\mathrm{\pi}$*r*_{1}^{2} = $\mathrm{\pi}$(24)^{2}

= 576$\mathrm{\pi}$ cm^{2}

Area of second circle = $\mathrm{\pi}$*r*_{2}^{2} = $\mathrm{\pi}$(7)^{2}

= 49$\mathrm{\pi}$ cm^{2}

Given that,

Area of the circle = Area of first circle + Area of second circle

∴ $\mathrm{\pi}$*R*^{2} = 576$\mathrm{\pi}$ + 49$\mathrm{\pi}$ (where *R* is the radius of the new circle)

⇒ $\mathrm{\pi}$*R*^{2} = 625$\mathrm{\pi}$

⇒ *R*^{2} = 625

⇒ *R* = 25 (∵ radius cannot be negative)

∴ Radius of the circle = 25 cm

Thus, diameter of the circle = 2*R* = 50 cm.

Hence, the correct answer is option (D).

#### Page No 122:

#### Question 1:

Is the area of the circle inscribed in a square of side *a* cm, $\mathrm{\pi}$*a*^{2} cm^{2}? Give reasons for your answer.

#### Answer:

False

Let *a* be the side of square.

Given that, the circle is inscribed in the square.

∴ Diameter of circle = Side of square = *a*

⇒ Radius of the circle = $\frac{a}{2}$

⇒ Area of the circle = $\mathrm{\pi}{\left(\frac{a}{2}\right)}^{2}=\mathrm{\pi}\frac{{a}^{2}}{4}$

Hence, area of the circle is $\mathrm{\pi}\left(\frac{{a}^{2}}{4}\right)$ cm^{2}.

#### Page No 122:

#### Question 2:

Will it be true to say that the perimeter of a square circumscribing a circle of radius *a* cm is 8*a* cm? Give reasons for your answer.

#### Answer:

True

Let radius of circle *r* = *a* cm.

∴ Diameter of the circle = *d*

= 2 × Radius

= 2*a* cm

As, the circle is inscribed in the square.

Therefore,

Side of a square = Diameter of circle = 2*a* cm

Hence,

Perimeter of a square = 4 × (side)

= 4 × 2*a*

= 8*a* cm

#### Page No 122:

#### Question 3:

In Fig 11.3, a square is inscribed in a circle of diameter *d* and another square is circumscribing the circle. Is the area of the outer square four times the area of the inner square? Give reasons for your answer.

#### Answer:

False

Let the diameter of the circle be *d*.

Therefore, the diagonal of inner square EFGH is equal to the side of the outer square ABCD.

Also, the side of the outer square ABCD is equal to the diameter of circle, i.e., *d*.

Let the side of the inner square EFGH be *a*.

Now in right angled triangle $\u25b3$EFG, by Pythagoras theorem,

(EG)^{2} = (EF)^{2} + (FG)^{2}

⇒ *d*^{2} = *a*^{2} +*a*^{2}

⇒ *d*^{2} = 2*a*^{2}

⇒ *a*^{2} = $\frac{{d}^{2}}{2}$

∴ Area of inner square = *a*^{2} = $\frac{{d}^{2}}{2}$

Also, the area of outer square is *d*^{2}. Thus, the area of the outer square is only two times the area of the inner square.

Hence, area of outer square is not equal to four times the area of the inner square.

#### Page No 123:

#### Question 4:

Is it true to say that area of a segment of a circle is less than the area of its corresponding sector? Why?

#### Answer:

False

It is not true because in case of major segment, area is always greater than the area of its corresponding sector. It is true only in the case of minor segment.

#### Page No 123:

#### Question 5:

Is it true that the distance travelled by a circular wheel of diameter *d* cm in one revolution is 2$\mathrm{\pi}$*d* cm? Why?

#### Answer:

False

The distance travelled by a circular wheel of radius *r* in one revolution is equal to the circumference of the circle.

Circumference of the circle = $\mathrm{\pi}$*d*, where *d* is the diameter of the circle.

#### Page No 123:

#### Question 6:

In covering a distance *s* metres, a circular wheel of radius *r* metres makes $\frac{s}{2\mathrm{\pi}r}$ revolutions. Is this statement true? Why?

#### Answer:

True

The distance travelled by a circular wheel of radius *r* m in one revolution is equal to the circumference of the circle i.e. 2$\mathrm{\pi}$*r*.

∴ No. of revolutions completed in 2$\mathrm{\pi}$*r* m distance = 1

No. of revolutions completed in 1 m distance = $\left(\frac{1}{2\mathrm{\pi}r}\right)$

No. of revolutions completed in *s* m distance = $\left(\frac{1}{2\mathrm{\pi}r}\right)$ $\times s$ .

#### Page No 123:

#### Question 7:

The numerical value of the area of a circle is greater than the numerical value of its circumference. Is this statement true? Why?

#### Answer:

False

Let *r* be the radius of the circle.

Area of the circle = $\mathrm{\pi}$*r*^{2}

Circumference of the circle = 2$\mathrm{\pi}$*r*

Case 1 :

Both are equal only when *r* = 2.

Case 2 :

Numerical value of circumference is greater than numerical value of area of circle when 0 < *r* < 2 .

Case 3 :

Numerical value of area of circle is greater than the numerical value of the circumference of the circle when *r* > 2 .

#### Page No 123:

#### Question 8:

If the length of an arc of a circle of radius *r* is equal to that of an arc of a circle of radius 2*r*, then the angle of the corresponding sector of the first circle is double the angle of the corresponding sector of the other circle. Is this statement false? Why?

#### Answer:

True.

Let P and Q be two circles with radius *r* and 2*r* respectively. Let C_{1} and C_{2} be the centers of the circles P and Q respectively.

Let AB be the arc length of P and CD be the arc length of Q.

Let *θ*_{1} and *θ*_{2} be the angle subtended by the arc AB and CD respectively on the center.

Given that, AB = CD = *l* (say) .....(1)

Now, arc length = $\frac{{\theta}_{1}}{360\xb0}\times 2\mathrm{\pi}\left(\mathrm{radius}\right)$

$\therefore \mathrm{arc}\left(\mathrm{AB}\right)=\frac{{\theta}_{1}}{360\xb0}\times 2\mathrm{\pi}r$

and

$\begin{array}{rcl}\therefore \mathrm{arc}\left(\mathrm{CD}\right)& =& \frac{{\theta}_{2}}{360\xb0}\times 2\mathrm{\pi}\left(2r\right)\\ & =& \frac{{\theta}_{2}}{360\xb0}\times 4\mathrm{\pi}r\end{array}$

$\Rightarrow \frac{{\theta}_{1}}{360\xb0}\times 2\mathrm{\pi}r=\frac{{\theta}_{2}}{360\xb0}\times 4\mathrm{\pi}r$ [Using (1) ]

$\Rightarrow {\theta}_{1}=2{\theta}_{2}$

∴ Angle of the corresponding sector of the first circle is double the angle of the corresponding sector of the other circle.

#### Page No 123:

#### Question 9:

The areas of two sectors of two different circles with equal corresponding arc lengths are equal. Is this statement true? Why?

#### Answer:

False

It is true for arcs of the same circle. But it does not hold true in different circles.

#### Page No 123:

#### Question 10:

The areas of two sectors of two different circles are equal. Is it necessary that their corresponding arc lengths are equal? Why?

#### Answer:

False

It is true for arcs of the same circle. But in different circles, it does not hold true.

#### Page No 123:

#### Question 11:

Is the area of the largest circle that can be drawn inside a rectangle of length *a *cm and breadth* b* cm (*a* >* b*) is $\mathrm{\pi}$*b*^{2} cm^{2}? Why?

#### Answer:

False

The largest circle that can be drawn inside a rectangle is possible when,

Diameter of the circle = Breadth of the rectangle = *b*

∴ Radius of the circle = $\frac{b}{2}$

Hence, area of the circle = $\mathrm{\pi}$*r ^{2}* = $\mathrm{\pi}{\left(\frac{b}{2}\right)}^{2}$.

#### Page No 123:

#### Question 12:

Circumferences of two circles are equal. Is it necessary that their areas be equal? Why?

#### Answer:

True

We are given that

Circumference of circle with radius *R*_{1} = Circumference of circle with radius *R*_{2}

⇒ 2$\mathrm{\pi}$*R*_{1} = 2$\mathrm{\pi}$*R*_{2}

⇒ *R*_{1} = *R*_{2}

⇒ $\mathrm{\pi}$(*R*_{1})^{2} = $\mathrm{\pi}$(*R*_{2})^{2}

Hence the areas are also equal.

#### Page No 123:

#### Question 13:

Areas of two circles are equal. Is it necessary that their circumferences are equal? Why?

#### Answer:

True

We are given that,

Area of circle with radius *R*_{1} = Area of circle with radius *R*_{2}

⇒ $\mathrm{\pi}$(*R*_{1})^{2} = $\mathrm{\pi}$(*R*_{2})^{2}

⇒ *R*_{1} = *R*_{2}

⇒ 2$\mathrm{\pi}$*R*_{1}= 2$\mathrm{\pi}$*R*_{2}

Hence, the circumferences are also equal.

#### Page No 123:

#### Question 14:

Is it true to say that area of a square inscribed in a circle of diameter *p* cm is *p*^{2} cm^{2}? Why?

#### Answer:

False

When the square is inscribed in the circle, the diameter of a circle is equal to the diagonal of a square but not the side of the square.

Let side of square = *a*

∴ *p*^{2} = *a*^{2} + *a*^{2}

⇒ *p*^{2} = 2*a*^{2}

$\Rightarrow {a}^{2}=\frac{{p}^{2}}{2}$

Hence, area of square is $\frac{{p}^{2}}{2}$.

#### Page No 125:

#### Question 1:

Find the radius of a circle whose circumference is equal to the sum of the circumferences of two circles of radii 15 cm and 18 cm.

#### Answer:

Radius of first circle = *r*_{1} = 15 cm

Radius of second circle = *r*_{2} = 18 cm

∴ Circumference of first circle = 2$\mathrm{\pi}$*r*_{1} = 30$\mathrm{\pi}$ cm

Circumference of second circle = 2$\mathrm{\pi}$*r _{2}* = 36$\mathrm{\pi}$ cm

Let

*R*be the radius of the circle.

Given that,

Circumference of circle = Circumference of first circle + Circumference of second circle

⇒ 2$\mathrm{\pi}$

*R*= 2$\mathrm{\pi}$

*r*

_{1}+ 2$\mathrm{\pi}$

*r*

_{2}

⇒ 2$\mathrm{\pi}$

*R*= 30$\mathrm{\pi}$ + 36$\mathrm{\pi}$

⇒ 2

*R*= 66

⇒

*R*= 33 cm

Hence, required radius of a circle is 33 cm.

#### Page No 125:

#### Question 2:

In Fig. 11.5, a square of diagonal 8 cm is inscribed in a circle. Find the area of the shaded region.

#### Answer:

Let *a* be the side of square.

∴ Diameter of a circle = Diagonal of the square = 8 cm

Now in right angled triangle $\u25b3$ABC, by Pythagoras theorem,

(AC)^{2} = (AB)^{2} + (BC)^{2}

⇒ (8)^{2} = *a*^{2} +*a*^{2}

⇒ 64 = 2*a*^{2}

⇒ *a*^{2} = 32

Hence, the area of square = *a*^{2} = 32 cm^{2}

⇒ Radius of the circle = 4 cm

∴ Area of the circle = $\mathrm{\pi}$*r*^{2}

= $\mathrm{\pi}$(4)^{2}

= 16$\mathrm{\pi}$ cm^{2}

So, the area of the shaded region = Area of circle – Area of square

∴ the area of the shaded region = 16$\mathrm{\pi}$ – 32

= 18.286 cm^{2}

#### Page No 126:

#### Question 3:

Find the area of a sector of a circle of radius 28 cm and central angle 45°.

#### Answer:

Area of a sector of a circle = $\frac{\theta}{360\xb0}\times \mathrm{\pi}\times {r}^{2}$ ,

where *r* is the radius and *θ* the angle subtended by the arc at the center of the circle.

Here, radius of circle = 28 cm,

Angle subtended at the center = 45°

∴ Area of a sector of a circle = $\frac{45\xb0}{360\xb0}\times \mathrm{\pi}\times {\left(28\right)}^{2}$

= 308 cm^{2}

Hence, the required area of a sector of a circle is 308 cm^{2}.

#### Page No 126:

#### Question 4:

The wheel of a motor cycle is of radius 35 cm. How many revolutions per minute must the wheel make so as to keep a speed of 66 km/h?

#### Answer:

Radius of wheel = *r* = 35 cm

Now,

1 revolution of the wheel = Circumference of the wheel = 2$\mathrm{\pi}$*r*

= $2\times \left(\frac{22}{7}\right)\times 35$

= 220 cm

But speed of the wheel = 66 km/h

= $\frac{66\times 1000\times 100}{60}\mathrm{cm}/\mathrm{min}$

= 110000 cm/min

∴ Number of revolutions in 1 min = $\frac{110000}{220}$ = 500

Hence, required number of revolutions per minute is 500.

#### Page No 126:

#### Question 5:

A cow is tied with a rope of length 14 m at the corner of a rectangular field of dimensions 20 m × 16 m. Find the area of the field in which the cow can graze.

#### Answer:

Let ABCD be a rectangular field.

Length of field = 20 m

Breadth of the field = 16 m

Suppose a cow is tied at a point A.

Let length of rope be AE = 14 m = *l* (say).

Now,

Angle subtended at the center of the sector = 90°

∴ Area of a sector of a circle = $\frac{90\xb0}{360\xb0}\times \mathrm{\pi}\times {\left(14\right)}^{2}$

= 154 m^{2}

Hence, the required area of a sector of a circle is 154 m^{2}.

#### Page No 126:

#### Question 6:

Find the area of the flower bed (with semi-circular ends) shown in Fig. 11.6.

#### Answer:

The area of the flower bed is given as:

Area of the flower bed = Area of the rectangular portion + Area of the two semi-circles.

Length and breadth of the rectangular portion AFDC of the flower bed are 38 cm and 10 cm respectively.

∴ Area of rectangle AFDC = Length × Breadth

= 38 × 10

= 380 cm^{2}

Both ends of flower bed are semi-circle in shape.

∴ Diameter of the semi-circle = Breadth of the rectangle AFDC = 10 cm

⇒ Radius of the semi circle = $\frac{10}{2}$ = 5 cm

Now,

Area of the semi-circle = $\left(\frac{{\mathrm{\pi r}}^{2}}{2}\right)$

= $\frac{25\mathrm{\pi}}{2}$ cm^{2}

Since there are two semi-circles in the flower bed,

∴ Area of two semi-circles = 2 × $\left(\frac{{\mathrm{\pi r}}^{2}}{2}\right)$ = 25$\mathrm{\pi}$ cm^{2}

Total area of flower bed = (380 + 25$\mathrm{\pi}$) cm^{2}.

#### Page No 126:

#### Question 7:

In Fig. 11.7, AB is a diameter of the circle, AC = 6 cm and BC = 8 cm. Find the area of the shaded region (Use π= 3.14).

#### Answer:

Given, AC = 6 cm and BC = 8 cm

The diameter of a circle subtends right angle at any other point on the circle. Thus, $\u25b3$ABC is a right angled triangle.

Therefore, in $\u25b3$ACB,

(AB)^{2} = (AC)^{2} + (CB)^{2} (By Pythagoras theorem)

⇒ (AB)^{2} = (6)^{2} + (8)^{2}

⇒ (AB)^{2} = 36 + 64

⇒ (AB)^{2} = 100

⇒ AB = 10

∴ Diameter of the circle = 10 cm

Thus, the radius of the circle is 5 cm.

Area of circle = $\mathrm{\pi}$*r ^{2}*

= $\mathrm{\pi}$(5)

^{2}

= 25$\mathrm{\pi}$ cm

^{2}

= 25 × 3.14 cm

^{2}

= 78.5 cm

^{2}

Also,

Area of the right angled triangle = $\left(\frac{1}{2}\right)$ × Base × Height

= $\left(\frac{1}{2}\right)$ × AC × CB

= $\left(\frac{1}{2}\right)$ × 6 × 8

= 24 cm

^{2}

Now,

Area of the shaded region = Area of the circle – Area of the triangle

= (78.5 $-$ 24) cm

^{2}

= 54.5 cm

^{2}

#### Page No 126:

#### Question 8:

Find the area of the shaded field shown in Fig. 11.8.

#### Answer:

The figure comprises of a rectangle and a semi-circle.

Now,

area of the figure = Area of the semi-circle + Area of the rectangle

Here, from the figure, radius of the semi-circle = *r* = 6 $-$ 4 = 2 m

∴ Area of the semi-circle = $\frac{\mathrm{\pi}{r}^{2}}{2}$

= $\frac{4\mathrm{\pi}}{2}$

= 2$\mathrm{\pi}$

Also, area of the rectangle = Length × Breadth

= AB × BC

= 4 × 8

= 32 m^{2}

∴ Area of shaded region = Area of rectangle ABCD + Area of semi-circle DEF

= (32 + 2$\mathrm{\pi}$) m^{2}

#### Page No 127:

#### Question 9:

Find the area of the shaded region in Fig. 11.9.

#### Answer:

Area of the shaded region = Area of the rectangle ABCD – (Area of the rectangle EFGH + (Area of the semi-circle EFJ + Area of the semi-circle GHI)

Length and breadth of outer rectangle ABCD are 26 m and 12 m respectively.

∴ Area of the rectangle ABCD = Length × Breadth

= AB × BC

= 26 × 12

= 312 m^{2}

From the figure, length and breadth of inner rectangle EFGH are (26 − 5 − 5) m and (12 − 4 − 4) m, i.e., 16 m and 4 m respectively.

∴ Area of the rectangle EFGH = Length × Breadth

= EF × FG

= 16 × 4

= 64 m^{2}

Breadth of the inner rectangle = Diameter of the semi-circle EJF = *d* = 4 m

∴ Radius of semi-circle EJF = *r* = 2 m

Area of the semi-circle EFJ = Area of the semi-circle GHI

= $\frac{{\mathrm{\pi r}}^{2}}{2}$

= $\frac{4\mathrm{\pi}}{2}$

= 2$\mathrm{\pi}$ m^{2}

∴ Area of shaded region = 312 – (64 + 2$\mathrm{\pi}$ + 2$\mathrm{\pi}$) m^{2}

= (248 – 4$\mathrm{\pi}$) m^{2}

#### Page No 127:

#### Question 10:

Find the area of the minor segment of a circle of radius 14 cm, when the angle of the corresponding sector is 60°.

#### Answer:

Let *r* be the radius of the circle = 14 cm.

Angle subtended at the center of the sector = *θ* = 60°

In $\u25b3$AOB,

∠AOB = 60°,

∠OAB = ∠OBA = *θ*

∴ *θ* + *θ* + 60 = 180° (∵ sum of all interior angles of a triangle is 180°)

⇒ 2*θ* = 120°

⇒ *θ* = 60°

∴ Each angle is of 60° and hence, $\u25b3$AOB is an equilateral triangle.

Now,

Area of the minor segment = Area of the sector AOBC – Area of triangle AOB

Angle subtended at the center of the sector = 60°

Angle subtended at the center (in radians) = *θ* = $\frac{60\mathrm{\pi}}{180}$ = $\frac{\mathrm{\pi}}{3}$

∴ Area of a sector of a circle = $\frac{{r}^{2}\theta}{2}$

= $\frac{1}{2}\times {\left(14\right)}^{2}\times \frac{\mathrm{\pi}}{3}$

= $\frac{308}{3}$ cm^{2}

Area of the equilateral triangle = $\frac{\sqrt{3}}{4}{\left(\mathrm{side}\right)}^{2}$

= $\frac{\sqrt{3}}{4}{\left(14\right)}^{2}$

= $49\sqrt{3}$ cm^{2}

∴ Area of minor segment = $\left(\frac{308}{3}-49\sqrt{3}\right)$ cm^{2}.

#### Page No 127:

#### Question 11:

Find the area of the shaded region in Fig. 11.10, where arcs drawn with centres A, B, C and D intersect in pairs at mid-points P, Q, R and S of the sides AB, BC, CD and DA, respectively of a square ABCD (Use π = 3.14).

#### Answer:

Since P, Q, R and S are mid-points of AB, BC, CD and DA respectively.

∴ AP = PB = BQ = QC = CR = RD = DS = SA = 6 cm.

Given that,

side of a square BC = 12 cm

Area of the square = 12 × 12 = 144 cm^{2}

Now,

Area of the shaded region = Area of the square $-$ (Area of the four quadrants)

Here,

Area of one quadrant = $\frac{{\mathrm{\pi r}}^{2}}{4}=\frac{3.14\times {\left(6\right)}^{2}}{4}=\frac{113.04}{4}$ cm^{2}

Area of four quadrants = 113.04 cm^{2}

∴ Area of the shaded region = 144 − 113.04

= 30.96 cm^{2}

#### Page No 127:

#### Question 12:

In Fig. 11.11, arcs are drawn by taking vertices A, B and C of an equilateral triangle of side 10 cm. to intersect the sides BC, CA and AB at their respective mid-points D, E and F. Find the area of the shaded region (Use π = 3.14).

#### Answer:

Since D, E, F bisects BC, CA, AB respectively.

∴ AE = EC = CD = DB = BF = FA = 5 cm

Now area of the shaded region = (Area of the three sectors)

Since the triangle is an equilateral triangle, therefore each angle is of 60°.

∴ Angle subtended at the center of each sector = 60°

Angle subtended at the center (in radians) = *θ* = $\frac{60\times \mathrm{\pi}}{180}$ = $\frac{\mathrm{\pi}}{3}$

Radius of each sector = 5 cm

∴ Area of a sector of a circle

= $\frac{1}{2}\times {r}^{2}\times \theta $

= $\frac{1}{2}\times {5}^{2}\times \frac{\mathrm{\pi}}{3}$

= $\frac{25\times 3.14}{6}$ cm^{2}

∴ Area of three sectors of a circle = $3\times \frac{78.5}{6}$ cm^{2}

= 39.25 cm^{2}

Thus, area of shaded region = 39.25 cm^{2}.

#### Page No 128:

#### Question 13:

In Fig. 11.12, arcs have been drawn with radii 14 cm each and with centres P, Q and R. Find the area of the shaded region.

#### Answer:

Let r be the radius of each sector = 14 cm

Area of the shaded region = Area of the three sectors

Let angles subtended at P, Q, R be *x*°, *y*°, *z*° respectively.

Angle subtended at P, Q, R (in radians) be $\frac{x\mathrm{\pi}}{180},\frac{y\mathrm{\pi}}{180},\frac{z\mathrm{\pi}}{180}$ respectively.

∴ Area of a sector with central angle at P

= $\frac{1}{2}\times {r}^{2}\times \theta $

= $\frac{1}{2}\times {\left(14\right)}^{2}\times \frac{x\mathrm{\pi}}{180}$

= $\frac{196\mathrm{\pi}x}{360}$ cm^{2}

Similarly,

Area of a sector with central angle at Q = $\frac{196\pi y}{360}$ cm^{2}

Area of a sector with central angle at R = $\frac{196\pi z}{360}$ cm^{2}

∴ Area of three sectors = $\left(\frac{196\mathrm{\pi}x}{360}+\frac{196\mathrm{\pi}y}{360}+\frac{196\mathrm{\pi}z}{360}\right)$ cm^{2} .

Since, sum of all interior angles in any triangle is 180°

∴ *x* + *y* + *z* = 180°

Thus, Area of three sectors

$=\left(\frac{196\pi}{360}\right)\times \left(x+y+z\right)\phantom{\rule{0ex}{0ex}}=\left(\frac{196\pi}{360}\right)\times \left(180\right)\phantom{\rule{0ex}{0ex}}=308{\mathrm{cm}}^{2}$

Hence, the required area of the shaded region is 308 cm^{2}.

#### Page No 128:

#### Question 14:

A circular park is surrounded by a road 21 m wide. If the radius of the park is 105 m, find the area of the road.

#### Answer:

Let *r* be the radius of the park = 105 m

Given that the circular park is surrounded by a road of width 21 m.

So, Radius of the outer circle (*R*) = (105+21) m = 126 m

Area of the road = Area of the outer circle – Area of the circular park

= π*R*^{2} – π*r*^{2}

= $\mathrm{\pi}\left\{{\left(126\right)}^{2}-{\left(105\right)}^{2}\right\}$

= $\frac{22}{7}\left(126+105\right)\times \left(126-105\right)$

= $\frac{22}{7}\times \left(231\right)\times \left(21\right)$

= $66\times 231$

= 15246 m^{2}

Hence, the required area of the road is 15246 m^{2}.

#### Page No 128:

#### Question 15:

In Fig. 11.13, arcs have been drawn of radius 21 cm each with vertices A, B, C and D of quadrilateral ABCD as centres. Find the area of the shaded region.

#### Answer:

Let *r* be the radius of each sector = 21 cm

Area of the shaded region = Area of the four sectors

Let angles subtended at A, B, C and D be *x*°, *y*°, *z*° and *w*° respectively.

Angle subtended at A, B, C, D (in radians) be $\frac{x\mathrm{\pi}}{180},\frac{y\mathrm{\pi}}{180},\frac{z\mathrm{\pi}}{180},\frac{\mathrm{w\pi}}{180}$.

∴ Area of a sector with central angle at A

= $\frac{1}{2}\times {r}^{2}\times \theta $

= $\frac{1}{2}\times {\left(21\right)}^{2}\times \frac{x\mathrm{\pi}}{180}$

= $\frac{441\mathrm{\pi}x}{360}$ cm^{2}

Similarly,

Area of a sector with central angle at B = $\frac{441\pi y}{360}$ cm^{2}

Area of a sector with central angle at C = $\frac{441\pi z}{360}$ cm^{2}

Area of a sector with central angle at D = $\frac{441\pi w}{360}$ cm^{2}

∴ Area of four sectors = $\left(\frac{441\pi x}{360}+\frac{441\pi y}{360}+\frac{441\pi z}{360}+\frac{441\pi w}{360}\right)$ cm^{2} .

Since, sum of all interior angles in any quadrilateral is 360°

∴ x + y + z +w = 360°

Thus, Area of four sectors

= $\left(\frac{441\pi}{360}\right)\times \left(x+y+z+w\right)=\left(\frac{441\pi}{360}\right)\times \left(360\right)$

= $441\mathrm{\pi}$ cm^{2}

= 1386 cm^{2}

Hence, the required area of the shaded region is 1386 cm^{2}.

#### Page No 128:

#### Question 16:

A piece of wire 20 cm long is bent into the form of an arc of a circle subtending an angle of 60° at its centre. Find the radius of the circle.

#### Answer:

Length of arc of circle = 20 cm

Here, the central angle $\theta =60\xb0$

$\therefore \mathrm{Length}\mathrm{of}\mathrm{arc}=\frac{\theta}{360}\times 2\mathrm{\pi}r$

$\Rightarrow 20=\frac{60}{360}\times 2\mathrm{\pi}r\phantom{\rule{0ex}{0ex}}\Rightarrow r=\frac{60}{\mathrm{\pi}}\mathrm{cm}$

Hence, the radius of the circle is $\frac{60}{\mathrm{\pi}}\mathrm{cm}$ .

#### Page No 132:

#### Question 1:

The area of a circular playground is 22176 m^{2}. Find the cost of fencing this ground at the rate of Rs 50 per metre.

#### Answer:

Given, Area of the circular playground = 22176 m^{2}

Let *r* be the radius of the circle.

∴ π*r*^{2} = 22176

⇒ $\left(\frac{22}{7}\right)$ *r*^{2} = 22176

⇒ *r*^{2} = 22176 × $\left(\frac{22}{7}\right)$

⇒* r*^{2} = 7056

⇒* r* = 84

∴ Radius of the circular playground = 84 m

Now, circumference of the circle = 2π*r*

= 2×$\left(\frac{22}{7}\right)$×84

= 528 m

Cost of fencing 1 meter of ground = ₹50

∴ Cost of fencing the total ground = ₹528 × 50 = ₹26400

#### Page No 132:

#### Question 2:

The diameters of front and rear wheels of a tractor are 80 cm and 2 m respectively. Find the number of revolutions that rear wheel will make in covering a distance in which the front wheel makes 1400 revolutions.

#### Answer:

Diameter of front wheels = *d*_{1} = 80 cm

Diameter of rear wheels = *d*_{2} = 2 m = 200 cm

Let *r*_{1} be the radius of the front wheels = $\frac{80}{2}$ = 40 cm

Let* r*_{2} be the radius of the rear wheels = $\frac{200}{2}$ = 100 cm

Now, Circumference of the front wheels = 2π*r*_{1}

= $2\times \frac{22}{7}\times 40$

= $\frac{1760}{7}$ cm

∴ Distance covered by the front wheel = 1400 × $\left(\frac{1760}{7}\right)$ = 352000 cm

Circumference of the rear wheels = 2π*r* = 2 × $\left(\frac{22}{7}\right)$ × 100 = $\frac{4400}{7}$ cm

Number of revolutions made by the rear wheel in covering a distance in which the front wheel makes 1400 revolutions

= $\frac{\mathrm{Distance}\mathrm{covered}\mathrm{by}\mathrm{front}\mathrm{wheel}}{\mathrm{Circumference}\mathrm{of}\mathrm{rear}\mathrm{wheel}}$

= $\frac{352000}{{\displaystyle \frac{4400}{7}}}$

$=\frac{24640}{44}\phantom{\rule{0ex}{0ex}}=560$

Hence, the rear wheel makes 560 revolutions.

#### Page No 132:

#### Question 3:

Sides of a triangular field are 15 m, 16 m and 17 m. With the three corners of the field a cow, a buffalo and a horse are tied separately with ropes of length 7 m each to graze in the field. Find the area of the field which cannot be grazed by the three animals.

#### Answer:

Sides of the triangle are 15 m, 16 m, and 17 m.

Now, perimeter of the triangle = (15+16+17) m = 48 m

∴ Semi-perimeter of the triangle = *s *= $\left(\frac{48}{2}\right)$ = 24 m

Area of the triangle = $\sqrt{24\times \left(24-15\right)\times \left(24-16\right)\times \left(24-17\right)}$ [ By Heron's Formula]

= 109.982 m^{2}

Let B, C and H be the corners of the triangle on which buffalo, cow and horse are tied respectively with ropes of 7 m each.

So, the area grazed by each animal will be in the form of a sector.

∴ Radius of each sector = *r* = 7 m

Let *x*, *y*, *z* be the angles at corners B, C, H respectively.

∴ Area of sector with central angle *x* = $\frac{x}{360\xb0}\times \mathrm{\pi}\times {r}^{2}$ = $\frac{x}{360\xb0}\times \mathrm{\pi}\times {\left(7\right)}^{2}$

∴ Area of sector with central angle *y* = $\frac{y}{360\xb0}\times \mathrm{\pi}\times {r}^{2}$ = $\frac{y}{360\xb0}\times \mathrm{\pi}\times {\left(7\right)}^{2}$

∴ Area of sector with central angle *z* = $\frac{z}{360\xb0}\times \mathrm{\pi}\times {r}^{2}$ = $\frac{z}{360\xb0}\times \mathrm{\pi}\times {\left(7\right)}^{2}$

Area of field not grazed by the animals = Area of triangle – (area of the three sectors)

= (109.982) $-$ $\left(\frac{x}{360\xb0}\times \mathrm{\pi}\times {\left(7\right)}^{2}+\frac{y}{360\xb0}\times \mathrm{\pi}\times {\left(7\right)}^{2}+\frac{z}{360\xb0}\times \mathrm{\pi}\times {\left(7\right)}^{2}\right)$

= (109.982) $-$ $\left(\frac{\mathrm{\pi}\times {\left(7\right)}^{2}}{360\xb0}\times \left(x+y+z\right)\right)$

= (109.982) $-$ $\left(\frac{\mathrm{\pi}\times {\left(7\right)}^{2}}{360\xb0}\times \left(180\xb0\right)\right)$

= 109.892 – 77 = 32.982 cm^{2}.

Hence, the area of the field which cannot be grazed by the three animals is 32.982 cm^{2}.

#### Page No 133:

#### Question 4:

Find the area of the segment of a circle of radius 12 cm whose corresponding sector has a central angle of 60° (Use π = 3.14).

#### Answer:

Radius of the circle = *r* = 12 cm

∴ OA = OB = 12 cm and ∠ AOB = 60° (Given)

Since, triangle OAB is an isosceles triangle, ∴ ∠ OAB = ∠ OBA = *θ* (say)

∴ *θ* + *θ* + 60° = 180° [Sum of interior angles of a triangle is 180°]

⇒2*θ* = 120°

⇒ *θ* = 60°

Thus, the triangle AOB is an equilateral triangle.

∴ AB = OA = OB = 12 cm

Area of the triangle AOB = $\frac{\sqrt{3}}{4}\times {a}^{2}$

= $\frac{\sqrt{3}}{4}\times {\left(12\right)}^{2}$

= $36\sqrt{3}$ cm^{2}

= 62.354 cm^{2}

Now, Central angle of the sector OBCA = 60°

Area of sector OBCA = $\frac{\mathrm{\pi}{r}^{2}}{360}\times \theta $

= $\frac{3.14\times 12\times 12}{360\xb0}\times 60\xb0$

= 75.36 cm^{2}

$\therefore $ Area of the segment ABCA

= Area of the sector OBCA – Area of the triangle AOB

= (75.36 – 62.354) cm^{2}

= 13.006 cm^{2}

Hence, the area of the segment of the circle is 13.006 cm^{2}.

#### Page No 133:

#### Question 5:

A circular pond is 17.5 m is of diameter. It is surrounded by a 2 m wide path. Find the cost of constructing the path at the rate of Rs 25 per m^{2}.

#### Answer:

Diameter of the circular pond = 17.5 m

Let *r* be the radius of the park = $\left(\frac{17.5}{2}\right)$ m = 8.75 m

Given that the circular pond is surrounded by a path of width 2 m.

So, Radius of the outer circle = *R* = (8.75 + 2) m = 10.75 m

Area of the road = Area of the outer circular path – Area of the circular pond

= π*R*^{2} – π*r*^{2}

= 3.14 × (10.75)^{2} – 3.14 × (8.75)^{2}

= 3.14 × ((10.75)^{2} – (8.75)^{2})

= 3.14 × ((10.75 + 8.75) × (10.75 – 8.75))

= 3.14 × 19.5 × 2 = 122.46 m^{2}

Thus, the area of the path is 122.46 m^{2}.

Now, Cost of constructing the path per m^{2} = Rs. 25

∴ Cost of constructing 122.46 m^{2} of the path = Rs. 25 × 122.46 = Rs. 3061.50

#### Page No 133:

#### Question 6:

In Fig. 11.17, ABCD is a trapezium with AB || DC, AB = 18 cm, DC = 32 cm and distance between AB and DC = 14 cm. If arcs of equal radii 7 cm with centres A, B, C and D have been drawn, then find the area of the shaded region of the figure.

#### Answer:

AB = 18 cm, DC = 32 cm

Distance between AB and DC, *h* = 14 cm

Now, Area of the trapezium

= $\left(\frac{1}{2}\right)$× (AB + DC) × *h*

= $\left(\frac{1}{2}\right)$ × (18+32) × 14

= 350cm^{2}

As AB ∥ DC,

∴ ∠ A +∠ D = 180°

and ∠ B +∠ C = 180°

Also, the radius of each arc = 7 cm

Therefore,

Area of the sector with central angle A = $\frac{1}{2}\times \left(\frac{\angle A}{180}\right)\times \mathrm{\pi}\times {r}^{2}$

Area of the sector with central angle D = $\frac{1}{2}\times \left(\frac{\angle D}{180}\right)\times \mathrm{\pi}\times {r}^{2}$

Area of the sector with central angle B = $\frac{1}{2}\times \left(\frac{\angle B}{180}\right)\times \mathrm{\pi}\times {r}^{2}$

Area of the sector with central angle C = $\frac{1}{2}\times \left(\frac{\angle C}{180}\right)\times \mathrm{\pi}\times {r}^{2}$

Total area of the sectors

= $\left(\frac{\angle A}{360}\right)\times \mathrm{\pi}\times {r}^{2}+\left(\frac{\angle D}{360}\right)\times \mathrm{\pi}\times {r}^{2}+\left(\frac{\angle \mathrm{C}}{360}\right)\times \mathrm{\pi}\times {r}^{2}+\left(\frac{\angle B}{360}\right)\times \mathrm{\pi}\times {r}^{2}$

= $\left(\frac{\angle A+\angle D}{360}\right)\times \mathrm{\pi}\times {r}^{2}+\left(\frac{\angle \mathrm{C}+\angle \mathrm{B}}{360}\right)\times \mathrm{\pi}\times {r}^{2}$

= $\left(\frac{180}{360}\right)\times \mathrm{\pi}\times {r}^{2}+\left(\frac{180}{360}\right)\times \mathrm{\pi}\times {r}^{2}$

= 77 + 77

= 154

∴ Area of shaded region = Area of trapezium – (Total area of sectors)

= 350 – 154

= 196 cm^{2}

Hence, the required area of the shaded region is 196 cm^{2}.

#### Page No 133:

#### Question 7:

Three circles each of radius 3.5 cm are drawn in such a way that each of them touches the other two. Find the area enclosed between these circles.

#### Answer:

The three circles are drawn in such a way that each of them touches the other two.

So, by joining the centers of the three circles, we get,

AB = BC = CA = 2(Radius) = 7 cm

Therefore, triangle ABC is an equilateral triangle with each side 7 cm.

∴ Area of the triangle = $\frac{\sqrt{3}}{4}\times {a}^{2}$

= $\frac{\sqrt{3}}{4}\times {7}^{2}$

= 21.2176 cm^{2}

Now, Central angle of each sector = 60°

Area of each sector = $\frac{60}{360}\times \mathrm{\pi}\times {\left(3.5\right)}^{2}$

= 6.4167 cm^{2}

Total area of three sectors = 3 × 6.4167 = 19.25 cm^{2}

∴ Area enclosed between three circles = Area of triangle ABC – Area of the three sectors

= 21.2176 – 19.25

= 1.9676 cm^{2}

Hence, the required area enclosed between these circles is 1.967 cm^{2} (approx).

#### Page No 133:

#### Question 8:

Find the area of the sector of a circle of radius 5 cm, if the corresponding arc length is 3.5 cm.

#### Answer:

Radius of the circle (*r*) = 5 cm

Arc length of the sector (*l*) = 3.5 cm

Let the central angle be *θ*.

$\Rightarrow \frac{\theta}{360}\times 2\mathrm{\pi}r=3.5\left(\mathrm{Arc}\mathrm{=}\frac{\theta}{360}\times 2\mathrm{\pi}r\right)$

$\Rightarrow \theta =3.5\times \frac{360}{10\mathrm{\pi}}$

$\mathrm{Area}\mathrm{of}\mathrm{circle}\mathrm{with}\mathrm{angle}\theta =\frac{\theta}{360}\times \mathrm{\pi}{r}^{2}$

= $3.5\times \frac{360}{10\mathrm{\pi}}\times \mathrm{\pi}\times \frac{{5}^{2}}{360}$

= 8.75 cm^{2}

Hence, the required area of the sector of a circle is 8.75 cm^{2}.

#### Page No 133:

#### Question 9:

Four circular cardboard pieces of radii 7 cm are placed on a paper in such a way that each piece touches other two pieces. Find the area of the portion enclosed between these pieces.

#### Answer:

The four circles are placed in such a way that each piece touches the other two pieces.

Now, on joining the centers of the circles by a line segment, we get a square ABDC with sides as,

AB = BD = DC = CA = 2(Radius) = 2(7) cm = 14 cm

Now, Area of the square = (Side)^{2} = (14)^{2} = 196 cm^{2}

∴ ∠ A = ∠ B = ∠ D = ∠ C = 90° [ABDC is a square]

Also, Radius of each sector = 7 cm

Area of each sector = $\frac{90}{360}\times \pi \times {7}^{2}$ [Area of sector = $\frac{\theta}{360}\times \mathrm{\pi}{r}^{2}$]

= $\frac{77}{2}$ cm^{2}

∴ Area of the shaded portion

= Area of square – Area of the four sectors

= $196-\left(4\times \frac{77}{2}\right)$

= 196 – 154

= 42 cm^{2}

Hence, required area of the portion enclosed between these pieces is 42 cm^{2}.

#### Page No 133:

#### Question 10:

On a square cardboard sheet of area 784 cm^{2}, four congruent circular plates of maximum size are placed such that each circular plate touches the other two plates and each side of the square sheet is tangent to two circular plates. Find the area of the square sheet not covered by the circular plates.

#### Answer:

Since, Area of the square = 784 cm^{2}

∴ Side of the square

= $\sqrt{\mathrm{Area}}$

= $\sqrt{784}$

= 28 cm

Since the four circular plates are congruent, therefore diameter of each circular plate = $\frac{28}{2}$ = 14 cm

∴ Radius of each circular plate = 7 cm

Area of the sheet not covered by plates = Area of the square – Area of the four circular plates

∴ Are of one circular plate = π*r*^{2} = 154 cm^{2}

So, Area of four plates = 4×154 = 616 cm^{2}

Area of the sheet not covered by plates = 784 – 616 = 168 cm^{2}.

#### Page No 133:

#### Question 11:

Floor of a room is of dimensions 5 m × 4 m and it is covered with circular tiles of diameters 50 cm each as shown in Fig. 11.18. Find the area of floor that remains uncovered with tiles. (Use π = 3.14)

#### Answer:

Length of the floor (*l*) = 5 m

Breadth of the floor (*b*) = 4 m

∴ Area of the floor = *l* × *b* = 5 × 4 = 20 m^{2}

Now, Diameter of each circular tile = 50 cm

∴ Radius of each circular tile (*r*) = 25 cm = 0.25 m

Area of one circular tile

= π*r*^{2}

= 3.14 × (0.25)^{2}

= 0.19625 m^{2}

Area of 80 such tiles

= 80 × 0.19625

= 15.7 m^{2}

Area of the floor that remains uncovered with tiles

= Area of the floor – Area of all 80 circular tiles

= 20 – 15.7

= 4.3 m^{2}

Hence, the required area of floor that remains uncovered with tiles is 4.3 m^{2}.

#### Page No 134:

#### Question 12:

All the vertices of a rhombus lie on a circle. Find the area of the rhombus, if area of the circle is 1256 cm^{2}. (Use π = 3.14).

#### Answer:

Area of the circle = 1256 cm^{2}

Let *r* be the radius of the circle.

∴ Area = π*r*^{2}

⇒ 1256 = 3.14 × *r*^{2}

⇒ *r*^{2} = $\frac{1256}{3.14}$ = 400

⇒ *r* = 20 cm

∴ Diameter of the circle (*d*) = 2 × 20 = 40 cm

Since all the vertices of a rhombus lie on a circle, therefore the diagonals of the rhombus pass through the center of the circle and thus diagonals of the rhombus are equal to the diameter of the circle.

Let *d*_{1} and *d*_{2} be the diagonal of the rhombus.

Since, diagonals of a rhombus are equal, therefore *d*_{1} = *d*_{2} = *d* = 40 cm

Now, area of the rhombus

= $\left(\frac{1}{2}\right)$ × *d*_{1} × *d*_{2}

= $\left(\frac{1}{2}\right)$ × 40 × 40

= 800 cm^{2}

Hence, the required area of rhombus is 800 cm^{2}.

#### Page No 134:

#### Question 13:

An archery target has three regions formed by three concentric circles as shown in Fig. 11.19. If the diameters of the concentric circles are in the ratio 1 : 2 : 3, then find the ratio of the areas of three regions.

#### Answer:

Diameters are in the ratio 1 : 2 : 3

So, let the diameters of the concentric circles be 2*r*, 4*r* and 6*r*.

∴ Radius of the circles be *r*, 2*r*, 3*r* respectively.

Now, Area of the outermost circle = π (Radius)^{2} = π (3*r*)^{2} = 9π*r ^{2}*

Area of the middle circle = π (Radius)

^{2}= π (2

*r*)

^{2}= 4π

*r*

^{2}

Area of the innermost circle = π (Radius)

^{2}= π (

*r*)

^{2}= π

*r*

^{2}

Now, area of the middle region

= Area of middle circle – Area of the innermost circle

= 4π

*r*

^{2 }$-$ π

*r*

^{2}

= 3π

*r*

^{2}

Now, area of the outer region

= Area of outermost circle – Area of the middle circle

= 9π

*r*

^{2}$-$ 4π

*r*

^{2}

= 5π

*r*

^{2}

Required ratio

= Area of inner circle : Area of the middle region : Area of the outer region

= π

*r*

^{2}: 3π

*r*

^{2}: 5π

*r*

^{2}

⇒ Required Ratio is 1 : 3 : 5.

#### Page No 134:

#### Question 14:

The length of the minute hand of a clock is 5 cm. Find the area swept by the minute hand during the time period 6 : 05 am and 6 : 40 am.

#### Answer:

Length of the minute hand = 5 cm = Radius of the clock

Minutes between the time period 6:05 am to 6:40 am = 35 minutes

In 60 minutes, the minute hand completes one revolution, i.e. 360°.

∴ Angle made by minute hand in 1 minute = $\frac{360\xb0}{60\xb0}$ = 6°

Thus angle made by minute hand in 35 minutes = 6° × 35 = 210°

∴ Area swept by minute hand in 35 minutes

= $\frac{210\xb0}{360\xb0}\times \mathrm{\pi}\times {r}^{2}$

= $\frac{210\xb0}{360\xb0}\times \mathrm{\pi}\times {\left(5\right)}^{2}$

= $\frac{275}{6}$

= 45.833 cm^{2}

Hence, the required area swept by the minute land is 45.833 cm^{2}.

#### Page No 134:

#### Question 15:

Area of a sector of central angle 200° of a circle is 770 cm^{2}. Find the length of the corresponding arc of this sector.

#### Answer:

Let *r* be the radius of the circle.

Given, Central angle = 200°

Area of the sector = 770 cm^{2}

$\Rightarrow 770=\frac{200\xb0}{360\xb0}\times \mathrm{\pi}\times {r}^{2}$

⇒ *r *= 21 cm

Thus, radius of the sector = 21 cm

Now, length of the corresponding arc

= $\frac{\theta}{360\xb0}\times 2\mathrm{\pi}r$

= $\frac{200\xb0}{360\xb0}\times 2\mathrm{\pi}\times 21$

= 73.33 cm

Hence, the required length of the corresponding arc is 73.33 cm.

#### Page No 135:

#### Question 16:

The central angles of two sectors of circles of radii 7 cm and 21 cm are respectively 120° and 40°. Find the areas of the two sectors as well as the lengths of the corresponding arcs. What do you observe?

#### Answer:

Radius of one sector (*r*_{1}) = 7 cm

Radius of second sector (*r*_{2}) = 21 cm

Central angle of one sector = 120°

Central angle of second sector = 40°

Area of first sector

= $\frac{\theta}{360\xb0}\times \mathrm{\pi}\times {\left({r}_{1}\right)}^{2}$

= $\frac{120\xb0}{360\xb0}\times \mathrm{\pi}\times {\left(7\right)}^{2}$

= $\frac{154}{3}$

= 51.33 cm^{2}

Area of second sector

= $\frac{\theta}{360\xb0}\times \mathrm{\pi}\times {\left({r}_{2}\right)}^{2}$

= $\frac{40\xb0}{360\xb0}\times \mathrm{\pi}\times {\left(21\right)}^{2}$

= 154 cm^{2}

Now, arc length of first sector

= $\frac{\theta}{360\xb0}\times \left(2\pi {r}_{1}\right)$

= $\frac{120\xb0}{360\xb0}\times \left(2\pi \times 7\right)$

= $\frac{44}{3}$ cm

Arc length of second sector

= $\frac{\theta}{360\xb0}\times \left(2\pi {r}_{2}\right)$

= $\frac{40\xb0}{360\xb0}\times \left(2\pi \times 21\right)$

= $\frac{44}{3}$ cm

Hence, we observe that arc lengths of two sectors of two different circles may be equal but their area need not be equal.

#### Page No 135:

#### Question 17:

Find the area of the shaded region given in Fig. 11.20.

#### Answer:

Area of square

= (side)^{2}

= 14^{2}

= 196 cm^{2}

As,

14 = 3 + *r* + 2*r* + *r* + 3

⇒ 14 = 6 + 4*r*

⇒ 14 – 6 = 4*r*

⇒ 8 = 4*r*

⇒ *r *= 2 cm

Area of internal portion = Area of 4 semi-circles + Area of inner square .....(1)

Area of semi-circle

= $\frac{{\mathrm{\pi r}}^{2}}{2}$

= $\frac{\pi {\left(2\right)}^{2}}{2}$

= $\frac{44}{7}$ cm^{2}

Area of 4 semicircles

= $4\times \frac{44}{7}$

= $\frac{176}{7}$ cm^{2}

Side of square = 2*r* = 2(2) = 4 cm

Area of square = 4^{2} = 16 cm^{2}

From (1), area of the internal portion = $\frac{176}{7}+16$

Area of shaded region

= Area of square $-$ Area of the internal portion

= $196-\left(\frac{176}{7}+16\right)$

= $\left(\frac{1084}{7}\right)$

= 154.85 cm^{2}

#### Page No 135:

#### Question 18:

Find the number of revolutions made by a circular wheel of area 1.54 m^{2} in rolling a distance of 176 m.

#### Answer:

Let *r* be the radius of the circular wheel.

Area = 1.54 m^{2}

∴ π*r*^{2} = 1.54

$\Rightarrow {r}^{2}=0.49$

⇒ *r* = 0.7 m

Thus, radius of the circular wheel = 0.7 m

Circumference of the wheel = 2π*r* = 2 × (3.14) × 0.7 = 4.4 m

Distance travelled by wheel in one revolution = Circumference of circular wheel = 4.4 m

Since, distance travelled by a circular wheel = 176 m

Total distance covered by the wheel = No. of revolutions made by wheel × (Distance covered in one revolution)

∴ No. of revolutions made by wheel

= $\frac{\mathrm{Total}\mathrm{Distance}}{\mathrm{Distance}\mathrm{in}\mathrm{one}\mathrm{revolution}}$

= $\frac{176}{4.4}$

= 40

Hence, the required number of revolutions made by a circular wheel is 40.

#### Page No 135:

#### Question 19:

Find the difference of the areas of two segments of a circle formed by a chord of length 5 cm subtending an angle of 90° at the centre.

#### Answer:

Length of the chord = 5 cm (Given)

Let *r* be the radius of the circle.

Then, OA = OB = *r* cm

Now, angle subtended at the center of the sector OABO = 90°

∴ Triangle AOB is a right-angled triangle.

So, by Pythagoras theorem, (AB)^{2} = (OA)^{2} + (OB)^{2}

⇒ 25 = 2*r*^{2}

$\Rightarrow r=\frac{5}{\sqrt{2}}$ cm

Area of ∆AOB = $\frac{1}{2}\times \mathrm{OA}\times \mathrm{OB}$

= $\frac{1}{2}\times \frac{5}{\sqrt{2}}\times \frac{5}{\sqrt{2}}$

= $\frac{25}{4}$ cm^{2}

Now, area of the minor sector

= $\frac{\theta}{360}\times \mathrm{\pi}\times {\mathrm{r}}^{2}$

= $\frac{90\xb0}{360\xb0}\times \mathrm{\pi}\times {\left(\frac{5}{\sqrt{2}}\right)}^{2}$

= $\left(\frac{25\mathrm{\pi}}{8}\right)$ cm^{2}

Area of the minor segment

= Area of the minor sector – Area of the isosceles triangle

= $\left(\frac{25\mathrm{\pi}}{8}-\frac{25}{4}\right)$ cm^{2}

Area of the major segment = Area of the circle – Area of the minor segment

= $\pi {r}^{2}-\left(\frac{25\mathrm{\pi}}{8}-\frac{25}{4}\right)$

= $\pi {\left(\frac{5}{\sqrt{2}}\right)}^{2}-\left(\frac{25\mathrm{\pi}}{8}-\frac{25}{4}\right)$

= $\left(\frac{25\mathrm{\pi}}{2}\right)-\left(\frac{25\mathrm{\pi}}{8}-\frac{25}{4}\right)$

= $\left(\frac{75\mathrm{\pi}}{8}+\frac{25}{4}\right)$ cm^{2}

∴ Difference of the areas of two segments of a circle

= Area of major segment – Area of minor segment|

= $\left|\left(\frac{75\mathrm{\pi}}{8}+\frac{25}{4}\right)-\left(\frac{25\mathrm{\pi}}{8}-\frac{25}{4}\right)\right|$

=$\left(\frac{25\mathrm{\pi}}{4}+\frac{25}{2}\right)$ cm^{2}

Hence, the required difference of the areas of two segments is $\left(\frac{25\mathrm{\pi}}{4}+\frac{25}{2}\right)$ cm^{2} .

#### Page No 135:

#### Question 20:

Find the difference of the areas of a sector of angle 120° and its corresponding major sector of a circle of radius 21 cm.

#### Answer:

Radius of the circle (*r*) = 21 cm

Area of the circle = $\mathrm{\pi}{r}^{2}$ = $\mathrm{\pi}{\left(21\right)}^{2}=1386$ cm^{2}

Central angle of the sector = 120°

Area of the minor sector

= $\frac{120\xb0}{360\xb0}\times \mathrm{\pi}\times {r}^{2}$

= $\frac{120\xb0}{360\xb0}\times \mathrm{\pi}\times {\left(21\right)}^{2}$

= 462 cm^{2}

Area of the major sector

= Area of the circle – Area of the sector

= 1386 – 462

= 924 cm^{2}

Now, difference of the areas of a sector and its corresponding major sector

= Area of major sector – Area of minor sector

= 924 $-$ 462

= 462 cm^{2}

Hence, the required difference of two sectors is 462 cm^{2}.

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