How to isolate the common ratio from a finite geometric series formula.

Question

For the formula $T = \frac{a\left(1-r^n\right)}{1-r}$ how do you isolate $r$ in terms of $T$, $a$, and/or $n$?

Answer 1

I do not believe it is possible (at least by any method that I know) except by a numerical method. I had a financial calculator that had these functions "built-in" and the handbook said that your question was calculated numerically.