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$3^{2n}$ is it arithmetic or geometric, or neither. If arithmetic find common difference and sum of first n terms. If geometric, find common ratio and sum of first n terms.

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    $\begingroup$ Try looking at the first three elements to find which it might be... $\endgroup$ – Thomas Andrews Mar 14 '13 at 19:21
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Hint: $3^{2n} = (3^2)^n = 9^n$.

$$ \sum_{k=1}^n r^k =\frac{r \left(r^n-1\right)}{r-1}$$

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