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

[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.

 

[micromass]

Use the formula with r^4 instead of r.

 

[jdinatale]

Use the formula with r^4 instead of r.

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