A circle is the set of points in a plane that are all the same distance from a fixed point in the plane. The fixed point is called the centre of the circle. The distance from the centre of the circle to the circle is called the radius of the circle.

Given the following:

r: The radius of a circle

C: The circumference of the circle

A: The area of the interior of the circle

C = 2πr     (explanation)

A = πr²     (explanation)