Circle equation

A circle is the set of all points in a plane
that are at a fixed distance, called the
radius, from a fixed point, called the
center. The standard form of the
equation of a circle with center (h, k) and
radius r is:
(x-h + (y- k =
This equation is derived directly from
the Pythagorean theorem applied to
any point (x, y) on the circle.