Answer
$-\ln(1+e^{-x})+C$
Work Step by Step
$\int \frac{e^{-x}}{1+e^{-x}}dx$
let $1+e^{-x}=u$
$-e^{—x}dx=du$
$\int \frac{e^{-x}}{1+e^{-x}}dx$
$=-\int \frac{1}{u}du$
$=-\ln(U)+C$
$=-\ln(1+e^{-x})+C$
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