# Homework Help: Find the sum

1. Mar 11, 2009

### wilcofan3

1. The problem statement, all variables and given/known data

Find the sum $$\sum_{0}^{\infty} \frac {n^2} {3^n}$$

2. Relevant equations
3. The attempt at a solution

I don't know how to go about finding this sum, I have a guess of what it will be just by adding the first ten terms or so, but how do I find an actual approximation?

2. Mar 11, 2009

### John Creighto

Try multiplying three geometric series together and see how close it is to the above series. I think it's simmilar to repeated roots in differential equations.

3. Mar 11, 2009

### wilcofan3

The only similar series I see here are:

$$\sum_{0}^{\infty} (\frac {1} {3})^n$$

$$\sum_{0}^{\infty} n^2$$

4. Mar 11, 2009

### foxjwill

Try looking at the second derivative of $$\sum_0^\infty x^n$$.

5. Mar 11, 2009

### lurflurf

homogeneous differentiation

sum=[(xD)^2](1/(1-x))|x=1/3

that is
sum=g(1/3)
when
f(x)=1/(1-x)
and
g(x)=x[x*f'(x)]'=(x^2)*f''(x)+x*f'(x)

Last edited: Mar 11, 2009