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## Homework Statement

Find [tex]\frac{dU_{ave}}{d\beta}[/tex]

where

[tex]U_{ave}=\sum_{k}\left(\frac{U_{k}exp(-U_{k}\beta)}{exp(-U_{k}\beta)}\right)[/tex]

## Homework Equations

## The Attempt at a Solution

My answer is supposed to be [tex]-(U_{ave}^{2})+(U_{ave})^{2}[/tex]

However I keep getting zero. I can only assume that the summation sign has something to do with the final form of the answer, but I have no idea how to deal with it. Can anyone offer a suggestion? Thanks