There is a problem that says: the series S(k) is a geometric series with S(2) = 425 and S(4) = 25. Find the different possible values for S(infinity).

So far I have seen that the common ratio is an imaginary number, and I think it is i (4/sqrt(17)), and I have to see what is the sum to infinity of the series by writing down some terms. There seems to be a connexion with Euler's series, but I really don't know what to do. Could somebody help?