# How do I get from here to here? - basic algebra

• March 3rd 2014, 06:53 PM
mathbeginner97
How do I get from here to here? - basic algebra
Hi!

I have no idea how to do this because the book I'm reading doesn't show me how:

A x sum of 1/(1+r)t where t goes on for however long, apparently equals A/r x [1 - 1/(1+r)t ]

how do you get there?

I'd be so thankful for someone to show me :))
• March 3rd 2014, 07:31 PM
Soroban
Re: How do I get from here to here? - basic algebra
Hello, mathbeginner97!

There is a typo . . .

Quote:

Show that: .

$A\sum^n_{t=1} \frac{1}{(1+x)^t} \;=\;\frac{A}{x}\left[1 - \frac{1}{(1+x)^n}\right]$

We have: . $S \;=\; \frac{A}{1+x} + \frac{A}{(1+x)^2} + \frac{A}{(1+x)^3} + \cdots + \frac{1}{(1+x)^n}$

This is geometric series with first term $a \,=\,\frac{A}{1+x},$
. . common ratio $r \,=\,\frac{1}{1+x}$ and $n$ terms.

The sum is: . $S \;=\;a\,\frac{1-r^n}{1-r}$

$S \;=\;\frac{A}{1+x}\left[\frac{1-(\frac{1}{1+x})^n}{1-(\frac{1}{1+x})}\right] \;=\;\frac{A}{1+x}\left[\frac{1 - \frac{1}{(1+x)^n}}{\frac{1+x-1}{1+x}}\right] \;=\;\frac{A}{1+x}\left[\frac{1-\frac{1}{(1+x)^n}}{\frac{x}{1+x}}\right]$

$S \;=\;A\left[\frac{1-\frac{1}{1+x)^n}}{x}\right] \;=\;\frac{A}{x}\left[1 - \frac{1}{(1+x)^n}\right]$
• March 3rd 2014, 07:40 PM
mathbeginner97
Re: How do I get from here to here? - basic algebra
Thank you very much!

I appreciate your help a lot! :)