Let (x1,y1) be the point on the circle x2+y2+2fy+λ=0 ⇒‌‌x12+y12+2fy1+λ=0 ⇒‌‌x12+y12+2fy1=−λ....(i) Now, the length of tangent from the point (x1,y1) to the circle x2+y2+2fy+µ=0 is =√(x12+y12+2fy1)+µ =√µ−λ  [from Eq. (i)]