# How do you find the integral of e^(-x)dx?

it's $- {e}^{- x} + c$
let $u = - x$ then $\frac{\mathrm{du}}{\mathrm{dx}} = - 1$ which means $\mathrm{dx} = - \mathrm{du}$
$\int \left({e}^{- x}\right) \mathrm{dx} = \int \left(- {e}^{u}\right) \mathrm{du} = - \int \left({e}^{u}\right) \mathrm{du} = - {e}^{u} + c = - {e}^{- x} + c$