Circle
A circle is the set of all points in a plane at a given distance (called the radius) from a given point (called the center.)
A line segment connecting two points on the circle and going through the center is called a diameter of the circle.
Clearly, if d represents the length of a diameter and r represents the length of a radius, then d = 2r.
The circumference C of a circle is the distance around the outside. For any circle, this length is related to the radius r by the equation
C = 2πr
where π (pronounced "pi") is an irrational constant approximately equal to 3.14.
The area of a circle is given the formula
A = πr2.
