CPM Homework Help : PC3 Problem 12-42

In the method shown in the Math Notes box, the series ended with $ar^{(n−1)}$ . Why is the power $\left(n − 1\right)$ and not $n$ , if there are $n$ terms in the sequence?

Hint:

Instead of going from $1$ to $n$ , the series goes from __ to $n − 1$ .