Equation of a 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.
Assume that are the coordinates of a point on the circle shown. The center is at , and the radius is .
Use the Distance Formula to find the equation of the circle.
Substitute and .
Square each side.
The equation of a circle with center and radius units is .