Answer
See below.
Work Step by Step
The sum of the first $n$ terms can be obtained by multiplying the first term($a_1$) by the difference of $1$ and the common ratio ($r$) on the power of $n$ divided by the difference of $1$ and the common ratio ($r$). So $S_n=\frac{a_1(1-r^n)}{1-r}$