## Geometry and Trigonometry## Plane Geometry |

## What is a circle? |

A circle is one of the most fundamental shapes in geometry—and one shape we commonly see every day. For mathematicians, a circle is defined as a set of points on a plane at a certain distance from a center point. In reality, a circle is a polygon with an infinite number of sides.

The distance of a line segment from the center to the points on the circle is called the *radius* (or a line segment whose endpoints are on any point on the circle and its center). A line segment that travels from one endpoint on the circle, through the radius (center of the circle), and to another endpoint directly opposite is called the *diameter;* two times the radius of a circle is the diameter. The outer perimeter of the circle is called the *circumference.* The *chord of a circle* is a line segment whose two endpoints are on the circle. *Concentric circles* are two or more circles that lie in the same plane and have the same center, but with different radii. Circles with the same radius are called *congruent circles.* (For more about circle measurements, see elsewhere in this chapter.)