Calculate Radius, Diameter, and Circumference

Some useful relations:

Diameter = 2 × radius

Circumference = 2 × π × radius

Also, circumference = π × diameter

Let's consider an example to look at the use of these relations.

Assume that the radius of a circle is 21 cm. Let's find the circumference of the circle.

Circumference = 2 × π × radius

                         = 2 × 3 × 21          [Using 3 as an estimated value for π]

                         = 6 × 21

                         = 126

So, the circumference of the circle is about 126 cm.