1. ## Sum to infinity

Hi, I've been doing an assignment for university and one of the questions is $((r^3)/(3^r))$ (sum from r=1 to infinity). I've searched all through my notes and on the internet but I can't find the answer anywhere! Help would be appreciated!
ebb

2. ## Re: Sum to infinity

You can use ratio test.

3. ## Re: Sum to infinity

I'm not trying to find whether or not it converges, I need to evaluate it. Unless the ratio test you mentioned is another one to the one I'm thinking of?

4. ## Re: Sum to infinity

To evaluate it, write
$r^3=r^2(r-1)+r^2=r(r-2)(r-1)+2r(r-1)+r^2=r(r-1)(r-2)+2r(r-1)+r(r-1)+r" alt="r^3=r^2(r-1)+r^2=r(r-2)(r-1)+2r(r-1)+r^2=r(r-1)(r-2)+2r(r-1)+r(r-1)+r" />.

5. ## Re: Sum to infinity

Okay, I will try that. Thank you!