Question

Find the standard of the circle which has a circle at ( 15, -20), radius 9.

Answer 1

$(15,-20), r=9\\ (x-x_0)^2+(y-y_0)^2=r^2\\ (x-15)^2+(y+20)^2=9^2\\ (x-15)^2+(y+20)^2=81$