Page 234 - ICSE Math 8

EXERCISE 21.5

1. Find the circumference of a circle with radius 21 cm.
2. Find the area of a circle with diameter 5.6 m.
3. Find the radius and area of a circular sheet whose circumference is 176 m.
2
4. The area of a circle is 616 cm . Find its radius and circumference.
2
5. A thin wire bent in the form of an equilateral triangle encloses an area of 16 3 m . If the same wire is
bent to form a circle, find the circumference of the circle.
6. A wheel has a diameter of 2.8 m. Find its circumference and the number of revolutions it will make to
cover 132 m.

7. Two circles have areas in the ratio 49 : 64. Find the ratio of their radii.
8. The inner and outer circumferences of a circular wheel are 154 cm and 88 cm. Find the width of the
wheel.

9. A circular flower bed is surrounded by a 4 cm wide path. The diameter of the flower bed is 36 cm.
Find the area of the path.
2
10. In a circular garden with circumference 17.6 m, there is space of 400 cm for one plant. Find the number
of plants that can be planted in the garden.
11. The diameter of a wheel is 2.1 m. Find the distance covered by it in 650 revolutions.
12. The radii of the inner and outer circumferences of a circular running track are 65 m and 72 m respectively.
Find the area of the track.
13. A motorcycle wheel with diameter 42 cm is making 60 revolutions in every 12 seconds. Find the speed
–1
of the motorcycle in km h .
14. Find the area of the shaded portion for each of the following. (Use π = 3.14)
(a) (b) (c)
1 cm

2 cm 2.5 cm
3 cm 4.5 cm
4 cm 24 cm

15. From a circular sheet of radius 14 cm, two circles of radii 3.5 cm and a rectangle of length 3 cm and
breadth 1 cm are removed, find the area of the remaining sheet.

AT A GLANCE

¾ The region enclosed by the boundary of a closed plane figure is known as its area.
¾ Area of a rectangle = length × breadth
¾ Area of a square = (side) 2
1
¾ Area of a triangle = × base × height = ss a− ( )(s b− )(s c− ) , where a, b and c are the sides of the
2
a + b + c
triangle and s = 2

3
2
¾ Area of an equilateral triangle = a , where a is the length of its side
4
¾ Area of a parallelogram = base × height
1
¾ Area of a rhombus = × product of the lengths of its diagonals
2

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