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I am wondering why the following equality holds ? $$\frac{1}{r^2} \frac {\partial}{\partial r}(r^2 \frac {\partial}{\partial r})= \frac {1}{r} \partial_r^2 r$$ . I tried making some substitutions but it doesn't seem to work . :(

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    For displayed equations, you need to put the punctuation inside the dollar signs (ideally spaced apart by `\;`), since, as you can see, otherwise it ends up on the following line.2012-11-06

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