Solution. We want to determine the integral of e^{-x}, i.e.:
\begin{align*} \int \frac{1}{e^x} dx = \int e^{-x}dx. \end{align*}
\begin{align*} \int e^{-x} dx &= - \int e^u du \\ &= -e^u + C \\ &= -e^{-x} + C. \end{align*}
\begin{align*} \int \frac{1}{e^x} dx = \int e^{-x}dx. \end{align*}
\begin{align*} \int e^{-x} dx &= - \int e^u du \\ &= -e^u + C \\ &= -e^{-x} + C. \end{align*}