A circle has center (0,0)(0, 0)(0,0) and radius rrr. If it passes through (x,y)(x, y)(x,y), then:
x2+y2=r2x^2 + y^2 = r^2x2+y2=r2
x+y=rx + y = rx+y=r
xy=r2xy = r^2xy=r2
x2−y2=r2x^2 - y^2 = r^2x2−y2=r2