Page 190 - Start Up Mathematics_5

Example 3: Find the radius of a circle whose circumference is 27 cm.

Solution: Circumference (C) = 27 cm

C 27
Diameter (d) = =
3.14 3.14
= 8.6 cm (approx.)

Radius (r) = 8.6 = 4.3 cm (approx.)
2

Maths Fun

Next time you buy a pizza make sure you have your ruler with you.
Measure the diameter of the pizza and find its circumference. You will
never be cheated. You will come to know if you have been given the
correct size of pizza or not.

EXERCISE 14.2

1. Draw a circle of radius 3.5 cm. Using a thread find its circumference. Also verify the
relation C = 3.14 × d (approximately).

2. There are 7 round tables at a party. The radius of each table is 20 cm. How much long
ribbon is required to cover these 7 tables?

3. The circumference of a compact disc is 28.26 cm. What is its radius?
4. The diameter of a cycle tyre is 30 cm. How far will it move in one turn of the wheel?

5. An asteroid hit the earth and created a huge round crater. Scientists measured the
distance around the crater as 52 km. What is the diameter of the crater?

6. The distance around the wheel of a truck is 282.6 cm. What is the radius of the
wheel?

LET’S EVALUATE

1. Find the diameter of the circles whose radii are given below.
(a) 4 cm (b) 3.5 cm (c) 7 cm

2. Find the radii of the circles whose diameters are given below.

(a) 32 dm (b) 54 mm (c) 8 m

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