Integral of $$$45 e^{- \frac{t}{20}}$$$

Find $$$\int 45 e^{- \frac{t}{20}}\, dt$$$.

Apply the constant multiple rule $$$\int c f{\left(t \right)}\, dt = c \int f{\left(t \right)}\, dt$$$ with $$$c=45$$$ and $$$f{\left(t \right)} = e^{- \frac{t}{20}}$$$:

$${\color{red}{\int{45 e^{- \frac{t}{20}} d t}}} = {\color{red}{\left(45 \int{e^{- \frac{t}{20}} d t}\right)}}$$

Let $$$u=- \frac{t}{20}$$$.

Then $$$du=\left(- \frac{t}{20}\right)^{\prime }dt = - \frac{dt}{20}$$$ (steps can be seen »), and we have that $$$dt = - 20 du$$$.

So,

$$45 {\color{red}{\int{e^{- \frac{t}{20}} d t}}} = 45 {\color{red}{\int{\left(- 20 e^{u}\right)d u}}}$$

Apply the constant multiple rule $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ with $$$c=-20$$$ and $$$f{\left(u \right)} = e^{u}$$$:

$$45 {\color{red}{\int{\left(- 20 e^{u}\right)d u}}} = 45 {\color{red}{\left(- 20 \int{e^{u} d u}\right)}}$$

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

$$- 900 {\color{red}{\int{e^{u} d u}}} = - 900 {\color{red}{e^{u}}}$$

Recall that $$$u=- \frac{t}{20}$$$:

$$- 900 e^{{\color{red}{u}}} = - 900 e^{{\color{red}{\left(- \frac{t}{20}\right)}}}$$

Therefore,

$$\int{45 e^{- \frac{t}{20}} d t} = - 900 e^{- \frac{t}{20}}$$

Add the constant of integration:

$$\int{45 e^{- \frac{t}{20}} d t} = - 900 e^{- \frac{t}{20}}+C$$

$$$\int 45 e^{- \frac{t}{20}}\, dt = - 900 e^{- \frac{t}{20}} + C$$$A