Answer
$\sum_{i=1}^{n}x^{i}$
Work Step by Step
We contract the sum into sigma notation by noting that it follows the pattern of a sum of powers of $x$ (e.g. $x^i$), starting with $x$ (e.g. $i=1$) and ending with $x^n$ (e.g. $i=n$):
$x+x^{2}+x^{3}+\displaystyle \cdots+x^{n}=\sum_{i=1}^{n}x^{i}$
