Does anyone recognize this equation ...

A = P(((1 + i)^{n}- 1)/((1-i) + (1 + i)^{n}))

It's supposed to calculate the future value (A) of an investment (P) after n years earning an annual interest i. If this equation is not correct, what is the correct equation. How is the above equation or the correct equation derived?

In financial analysis, the Rule of 72 provides that an investment P will double (A = 2P) invested at an interest rate (i) in 72/i years (n). For example, an investment P at 9% (i) will double (A = 2P) in 8 (n) years.

Plugging .09 for i and 8 for n in the above equation does not give A = 2P.

Steve