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## Homework Statement

Find the power series representation of f(x)= 1/((4+x)^2).

Use this representation to determine the sum of the series:

sum [1, infinity) of n*(3/8)^n.

I attached a screenshot of the problem for just in case.

## Homework Equations

## The Attempt at a Solution

What I did first was find the power series representation of f(x)= 1/(4+x).

I got sum [0, infinity) of ((-1)^n))(x^n)/(4^(n+1)).

and then I got the derivative of that which made it:

sum [1, infinity) of ((-1)^n))(n)(x^(n-1))/(4^(n+1)).

And so that is where I am lost. I don't know how to use that representation to find the sum of the series "n*(3/8)^n".