three concentric circlesv of which biggest is x^2+y^2=1,have their radii in A.P.if the line y=x+1 cuts all the circles in real and distinct points ,then the interval in which the common difference of A.P. will lie,is

Hii

You need to consider the radi in decreasing AP. The distance of the given line must be less than the smallest value of the radii.From there you will get relation between a and d.

From there you will get the interval of common difference.