Answer
$\sum_{i=1}^{n}(2i-1)$
Work Step by Step
We contract the sum into sigma notation by noting that it follows the pattern of a sum of odd integers (e.g. $2i-1$), starting with $1$ (e.g. $i=1$), and ending with $2n-1$ (e.g. $i=n$).
$1+3+5+7+\displaystyle \cdots+(2n-1)=\sum_{i=1}^{n}(2i-1)$