Quite frankly, I don't understand how this:
F = G * ( 1 / N - N / (1 + I)^N - 1 )
works at all, when *CLEARLY* the formula starts with 1/I
and *NOT* 1/N
Also, you are missing the *REQUIRED* parentheses around the sub-expression ( (1+I)^N - 1)
I think the only reason it *SEEMED* to work was because you used the *SAME NUMBER* (10) for both I and N!!!