1. Circles

2. Radius : The radius is a line segment with
one end point at the centre and the other
end on the circle. It is denoted by ‘r’. r O A
Diameter : Diameter is a chord passing through the centre of the circle. It is denoted by ‘d’. O A B d

3. Relationship between diameter and radius:
Diameter of a circle is twice of its radius.
Formula:
Diameter = 2 x radius
Radius = r O r A B d

4. Pi (π)
The ratio of circumference to diameter is constant for all circles. This ratio is called Pi and is written as π. Where π = C = π ⇒
C = πd ⇒
C = 2πr d 22 7

5. Example 1 : Find out the Diameter of a circle whose radius is 5 m.
Solution Given:- radius(r)= 5 To Find:- diameter(d) Diameter of a circle (d) = 2 x radius = 2 x 5 = 10m (ans)
Example 2 : Find out the radius of a circle whose diameter is 8 m.
Solution Given:- diameter(d) = 8 To Find:- radius(r)

6. Solution Given:- radius(r)= 21 To Find:- circumference(c)
Example 3 : Find out the circumference of a circle whose radius is 21 m.
Solution Given:- radius(r)= 21 To Find:- circumference(c)
Circumference (C) = 2πr 3
Divide 21 and 7 by 7
= 2 x 22 x 3
= 132 m (ans)
Example 4 : Find out the radius of a circle whose circumference is 44m.
Solution Given:- circumference(c) = 44 To Find:- radius(r)
Circumference (C) = 2πr
= 7 m (ans)
Divide 44 and 44 by 44

7. Solution Given:- circumference(c) = 44 To Find:- diameter(d)
Example 5: Find out the circumference of a circle whose diameter is 21 cm.
Solution Given:- diameter(d) = 21 To Find:- circumference(c)
Circumference (C) = πd 3
Divide 21 and 7 by 7
C = 22 x 3 = 66 cm (ans)
Example 6: Find out the diameter of a circle whose circumference is 44cm.
Solution Given:- circumference(c) = 44 To Find:- diameter(d)
Circumference (C) = πd 2
Divide 44 and 22 by 22
d = 2 x 7 = 14 cm (ans)

8. Try these
Find out the circumference of a circle whose diameter is 63 cm.
Find out the radius of a circle whose circumference is 66 m.