Writing an equation
 Use
 Substitute the values of the center in place of h and k.
 Substitute the values of the center of the circle in place of x and y.
 Simplify the expression to find r (the radius).
 Substitute the radius value into x^{2} + y^{2} = r^{2} when the center is (0,0).
 Substitute the radius and center (h, k) into (x  h)^{2} + (y  k)^{2} = r^{2} when the center is NOT (0,0).


