I'm working on a question like: find a simple g(n) (big theta) for $ f(n) = \sum _{i=1}^n 5^i $
My working starts with this $\frac{5-5^n}{1-5}$ which is not equivalent to the correct answer from wolfram alpha: $\frac{5}{4}\left(5^n-1\right)$. Step-by-step solutions aren't available for this problem type unfortunately.
(I know that this isn't the final answer for big theta but I'm stuck at this step and need to show how I reach the answer for g(n))
I'd been using the second equation from here on wikipedia.