(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

I have been shown this manipulation of a summation and I am wondering why the person could do it:

[tex] \sum_{k=n+1}^{0} -k 3^k [/tex]

Now change the variables with m=-k and we get:

[tex] \sum_{m=0}^{-n-1} m (\frac{1}{3})^m[/tex]

3. The attempt at a solution

I can see where the [tex] m (\frac{1}{3})^m[/tex] came from (just stick -m in for the k), but how did the summation change?

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# Homework Help: Changing variable in a sumation

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