The standard form of an equation of a circle with center at (h, k)
and radius r is .

(y − h)2 + (x − k)2 = r2

(x − h)2 − (y − k)2 = r2

(y + h)2 − (x + k)2 = r2

(x − h)2 + (y − k)2 = r2

(x + h)2 − (y + k)2 = r2