Step-by-step explanation:
A circle is a two dimensional geometric figure, a line, consisting of the set of all those points in a plane that are equal distant from a given which is the center of the circle.
A circle has angles from 0° to 360° ⇒ 0 to 2π. The distance between any point of a circle and its center is called radius.
A whole circle distance(circumference) will cover a 2π distance with respect with a equal distance from the center of the circle.