(d) The radical axis of the co-axial system of circles with limiting points A(1,2) and B(−2,1) is the perpendicular bisector of line joining limiting points A and B. ∵ Mid-point of A and B is (−
1
2
,
3
2
) and slope of perpendicular to AB is −3 ∴ Equation of required radical axis is y−