Let M and N be midpoints of PQ and ST respectively. ⇒MN is a radical axis of two circles C1:x2+y2=1.....(i) C2:(x−4)2+(y−1)2=r2 ⇒x2+y2−8x−2y+17−r2=0....(ii) From (i) and (ii); Equation of MN:8x+2y−18+r2=0 ⇒B is on x-axis ⇒B(
18−r2
8
,0) AB=√5 √(
18−r2
8
−4)2+1=√5 AB=√5 √(
18−r2
8
−4)2+1=√5 (By distance formed a) ⇒ On solving r2=2