# What is the derivative of e^(-x)?

To evaluate the $\left({e}^{-} x\right) '$ you must calculate the $\left({e}^{u}\right) ' \cdot u '$, where $u = - x$. But, $\left({e}^{u}\right) ' = {e}^{u}$, and $u ' = - 1$, and then:
$\left({e}^{-} x\right) ' = {e}^{-} x \cdot \left(- 1\right) = - {e}^{-} x$