From an external point P(x1,y1), two tangents can be drawn to a circle. The chord of contact (the line joining the two points of tangency) has equation T=0: xx1+yy1+g(x+x1)+f(y+y1)+c=0. The length of the chord of contact is 2rsqrt$\frac{S1}{(S1+r^2)}$... Actually, length = 2*sqrt(S1)*r/sqrt($d^2$) where d is distance from centre to P. More precisely, if L is the tangent length sqrt(S1) and d is distance from centre to P, then chord of contact length = 2rL/d.