# Homework Help: Evaluating a Series

1. Nov 7, 2011

### Ocasta

1. The problem statement, all variables and given/known data
First, I'd like to thank everybody a head of time. You guys have been an enormous help.

Second, I don't mind telling you that I'm finding sequences and series extremely frustrating. I usually pick this stuff up like nobody's business.

My problem is attached, but I will copy it down as I understand it.

$r = \frac{11}{24}$

$\sum _{i=1} ^\inf nr^n$

Mysteriously, this can be rewritten as
$\sum_{i=1} ^n ir^i = \frac{ nr^{n+2} - (n+1)r^{n+1} + r }{ (1 - r)^2 }$

2. Relevant equations
$\sum _{i=1} ^\inf nr^n \rightarrow n \sum _{i=1} ^\inf r^n \rightarrow n \frac{1}{1-r}$

3. The attempt at a solution

$\sum _{i=1} ^\inf nr^n \rightarrow n \sum _{i=1} ^\inf r^n \rightarrow n \frac{1}{1-r}$

$\frac{1}{1-r}$
This is a number greater than one,
$\frac{24}{13}$

So as n goes to infinity, the number just gets bigger and bigger right? Diverges to infinite is, apparently, not the answer.

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2. Nov 7, 2011

### Ray Vickson

You switched summation indices incorrectly. The series is either sum_{i} i*r^i or sum_{n} n*r^n; it is NOT sum_{i} n*r^n or whatever. Go back and read the question more carefully.
Remember: |r| = 11/24 is less than 1---that matters a lot.

RGV