5.5 The Substitution Rule/1

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \begin{align} & \int{e^{-x}} dx , u=-x \\[2ex] &= & u=-x \\ [2ex] & du=-dx\\ [2ex] &= \int{e^{-x}} dx \\ [2ex] &= -\int{e^{u}} du \\ [2ex] &= -e^{u} du \\ [2ex] &= -e^{-x}+C \\ [2ex] \end{align} }