Any way to figure out what this finite geometric series sums to?

1. Feb 23, 2012

jdinatale

I would like to find a nice formula for $\sum_{k=0}^{n - 1}ar^{4k}$. I know that $\sum_{k=0}^{n - 1}ar^{k} = a\frac{1 - r^n}{1 - r}$ and was wondering if there was some sort of analogue.

2. Feb 23, 2012

micromass

Use the formula with $r^4$ instead of r.

3. Feb 23, 2012

jdinatale

Thank you micromass for your help. Wow that was really simple.