The radical axis of two circles S1=0 and S2=0 is S1-S2=0 (a straight line). It is the locus of points having equal power with respect to both circles. Properties: (1) it is perpendicular to the line joining centres, (2) if circles intersect, the radical axis is their common chord, (3) if circles are tangent, it is the common tangent at the point of tangency. The radical centre of three circles is the intersection of any two radical axes and has equal power with respect to all three circles.