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

  1. Feb 23, 2012 #1
    I would like to find a nice formula for [itex]\sum_{k=0}^{n - 1}ar^{4k}[/itex]. I know that [itex]\sum_{k=0}^{n - 1}ar^{k} = a\frac{1 - r^n}{1 - r}[/itex] and was wondering if there was some sort of analogue.
  2. jcsd
  3. Feb 23, 2012 #2
    Use the formula with [itex]r^4[/itex] instead of r.
  4. Feb 23, 2012 #3
    Thank you micromass for your help. Wow that was really simple.
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