Page 134 - Maths Skills - 7

132 Maths

Similarly, perimeter of other regular polygons can be tabulated as follows:

Figures Name Formula Figures Name Formula

Pentagon P = 5a Heptagon P = 7a

Hexagon P = 6a Octagon P = 8a

Have you ever wondered what would be the perimeter of a circle? Do we need to measure it in the same way as
done for the irregular shapes? Let’s do an activity first.

CIRCUMFERENCE OF A CIRCLE
The distance around a circular region is known as its circumference. Tt is also called the perimeter of the circle
circumference of circle = 2pr
Circumference
= pd [Q d = 2r]
Radius
where, r = radius of the circle O A
(Centre)
d = diameter of the circle

Let’s Attempt

Example 1: If the diameter of a circle is 3 cm, find its circumference.
Solution: Circumference of a circle (C) = pd
Here, d = 3 cm

22 66
\ C = × 3 cm = cm = 9.43 cm
7 7
Example 2: The circumference of a circle is 44 cm. Find its radius and diameter.
Solution: Circumference of a circle (C) = 44 cm
Let ‘r’ be the radius, we know that C = 2pr

 222×  7
or, r = C ÷ 2p = 44 ÷  7  = 44 × 44
So, r = 7 cm  

Also diameter (d) = 2r = 2 × 7 = 14 cm.

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Example 3: Find the circumference of the circle, if its radius is 1 cm.
4
3 7
Solution: Radius of the circle (r) = 1 = cm
4 4
7
We know that C = 2pr = 2 × 22 × cm = 11 cm.
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