Computing interest rate from investment and return

At the beginning of every year a bank customer deposits v euros in an investment fund and withdraws, at the end of the n-th year, a capital of M euros. We want to compute the average yearly rate of interest r of this investment. Since M is related to r by the relation

$M = v\sum_{k=1}^n[(1 + r)^k] = v[(1 + r)/r][(1 + r)^n − 1]$ , we deduce that r is the root of the algebraic non-linear equation:

$f(r) = 0$, where $f(r) = M − v[(1+r)/r][(1 + r)^n − 1]$.

I just want to know that reviews are the problem not want to fix it, thanks for your support

-
"I just want to know that reviews are the problem" I'm sorry, it's not clear what you mean by this. Please edit. –  Gerry Myerson Aug 7 '12 at 3:16
Just fine. At this point a numerical method takes over. –  André Nicolas Aug 7 '12 at 3:16