What is the equation for finding the circumference of a circle from the radius?

circumference equal two times 'pi' times radius.

It is written algebraically as

C = 2pir

Since 2r = d (diameter) Then C = pi*d

Also the area (A) of a circle is

A = pi*r^(2)

'pi' is the constant of proportion of circles.

NB For school work pi = 3.14 or pi = 3.1416 or pi = 22/7