Consider the given circles are, S1≡(x−2)2+(y−3)2=a2 S2≡(x−5)2+(y−6)2=a2 The value of S1−S2 is calculated as (x−2)2+(y−3)2−(x−5)2−(y−6)2=a2−a2 (4−4x+9−6y)−(25−10x)−(36−12y)=0 6x+6y−48=0 x+y=8......(1) since the given circle cut orthogonally 2g1g2+2f1f2=c1+c2 2(−2)(−5)+2(−3)(−6)=((−2)2+(−3)2−a2)+((−5)2+(−6)2−a2) 20+36=(13−a2)+(61−a2) 56+74−2a2 a=3