Angle of intersection of two curves and the condition of orthogonality of two circles

Condition for two intersecting circles to be orthogonal

Let Circle 1 (termed as S1) be x² + y²+2g1x+2f1y+c = 0

And Circle 2 (termed as S2) be x² + y²+2g2x+2f2y+c = 0

Condition is 2(g1g2+f1f2) = c1+c2