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I'm trying to derive this

$ f(x)=\frac{6}{1+2e^{-5x}}$

and getting this

$ f'(x)=\frac{0(1+2e^{-5x})-6(0-10e^{-5x})}{(1+2e^{-5x})^2}$

$=\frac{60e^{-5x}}{(1+2e^{-5x})^2}$

But the answer I get when checking on Wolfram Alpha is

$f'(x)=\frac{60e^{5x}}{(2e^{5x}+2)^2}$

I don't understand how this works. How do the exponents for e become positive all of a sudden, and where does the +2 in the denominator come from?

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    Oops! The expression *must* have in the denominator $\,(e^{5x}+2)^2\,$...check this!2012-10-29

1 Answers 1

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You seemed to have copied the equation from Wolfram wrong (there is no $2$ in front of the $e^{5x}$). To see how the two expressions are the same multiply the top and bottom of the fraction by $(e^{5x})^2$.

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    Oh, the 2 sneaked it's way in there. Thanks to both for verifying and guiding. :)2012-10-29