$\int \frac{d x}{\left(1+e^{-x}\right)\left(1+e^{x}\right)}=?$

$\ln \left(1+e^{x}\right)$

$\frac{1}{1+e^{x}}$

$\frac{1}{1+e^{x}}+c$

$-\frac{1}{1+e^{x}}+c$