• A circle is the set of points in the plane that are at a fixed distance from a fixed point called the centre.
• This fixed distance is the radius.
• If the centre of the circle is (h, k) and its radius is r, then the point (x, y) lies on the circle if and only if:

(x − h)² + (y − k)² = r²

• This is the standard form of the equation of a circle. If the centre is the origin (0, 0), the equation becomes x² + y² = r².