it is similar to [this problem][1] whose solution is [here][2]. But what makes it interesting is the fact that the inversion about the circle at origin can be shifted by some amount ~~delta.So, ~~delta .So, you have to use the radii of the circles r3 and r4 for calculating the shift and backtranslate from the smaller circles to find the radii of the nth circle. The formulas can be found [here.][3].
My submission is [here.][4]. It is a really cool concept.
[1]: http://codeforces.com/problemset/problem/77/E
[2]: http://codeforces.com/blog/entry/1773
[3]: http://mathworld.wolfram.com/Inversion.html
[4]: http://www.codechef.com/viewsolution/7445907