To show the formula on the computer you need a program called LaTeX.

Here you can do the following:

A = P \frac{i(1+i)^n}{(1+i)^n-1} \Rightarrow P = A \frac{(1+i)^n - 1}{i(1+i)^n}

put [ m a t h] [/ m a t h] tags (without the spaces) around the above statement to get: