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Sum of Terms in a GP with General Starting Index
## Formula
For:
$$\sum_{k=m}^{n}ar^{k-1}$$
the sum is:
$$\sum_{k=m}^{n}ar^{k-1}=ar^{m-1}\frac{1-r^{n-m+1}}{1-r}$$
for:
$$r\ne1$$
## Usage
- Used to calculate partial sums of a GP beginning from any specified term.
