Integral of $$$y e^{- x}$$$ with respect to $$$x$$$

Find $$$\int y e^{- x}\, dx$$$.

Apply the constant multiple rule $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$ with $$$c=y$$$ and $$$f{\left(x \right)} = e^{- x}$$$:

$${\color{red}{\int{y e^{- x} d x}}} = {\color{red}{y \int{e^{- x} d x}}}$$

Let $$$u=- x$$$.

Then $$$du=\left(- x\right)^{\prime }dx = - dx$$$ (steps can be seen »), and we have that $$$dx = - du$$$.

The integral can be rewritten as

$$y {\color{red}{\int{e^{- x} d x}}} = y {\color{red}{\int{\left(- e^{u}\right)d u}}}$$

Apply the constant multiple rule $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ with $$$c=-1$$$ and $$$f{\left(u \right)} = e^{u}$$$:

$$y {\color{red}{\int{\left(- e^{u}\right)d u}}} = y {\color{red}{\left(- \int{e^{u} d u}\right)}}$$

The integral of the exponential function is $$$\int{e^{u} d u} = e^{u}$$$:

$$- y {\color{red}{\int{e^{u} d u}}} = - y {\color{red}{e^{u}}}$$

Recall that $$$u=- x$$$:

$$- y e^{{\color{red}{u}}} = - y e^{{\color{red}{\left(- x\right)}}}$$

Therefore,

$$\int{y e^{- x} d x} = - y e^{- x}$$

Add the constant of integration:

$$\int{y e^{- x} d x} = - y e^{- x}+C$$

$$$\int y e^{- x}\, dx = - y e^{- x} + C$$$A