Answer
$\sum_{k=0}^{n-1} ar^k$
Work Step by Step
We are given the sum:
$a+ar+ar^2+...+ar^{n-1}$
The general term of the sum is $ar^k$.
The number of terms is $n$, so the index $k$ goes from 0 to $n-1$.
We write the sum:
$a+ar+ar^2+...+ar^{n-1}=\sum_{k=0}^{n-1} ar^k$
