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$\frac{a}{1-r}$ I know that it should be (derivative of the top*bottom - top*derivative bottom) / (bottom squared).

$\frac{d}{dr}\frac{a}{1-r}$ I tried this got the answer wrong, and looked up how to do this and they showed:

$a \frac{d}{dr}\frac{1}{1-r}$ Why was the a pulled out and the derivative not taken on it? Is it because we are taking the derivative with respect to r? since a is not r we do nothing with it?