# Math Help - infinite series and interest

1. ## infinite series and interest

The above starts with S=1 + a^2 + a^3 + ... + a^n.

I understand how the first part (dealing with S) works but I dont understand how it relates to the expression below the paragraph.

$1-a^{n+1}$ in the second expression after the paragraph must correspond with $1- (1+\frac{r}{n})^{-nt}$ and $(1-a)$ must correspond to $(1-(1+\frac{r}{n})^{-1})$

but im not sure what role the first term: $x(1+\frac{r}{n})^{-1}$ plays or how we get to the third expression at the bottom.

2. ## Re: infinite series and interest

On the bottom, you do not have the equivalent of S-aS shown for the P equation. The answer lies in what would be on the left hand side.