If `S_1=x^2+y^2+2g_1x+2f_1y+c_1=0` and `S_2=x^2+y^2+2g_2x +2f_2y+c_2=0` are two circles with radii `r_1` and `r_2` respectively, show that the points at which the circles subtend equal angles lie on the circle `S_1/r_1^2=S_2/r_2^2`