Integral of $$$\frac{2 g r^{2} \sigma v}{9}$$$ with respect to $$$v$$$

Find $$$\int \frac{2 g r^{2} \sigma v}{9}\, dv$$$.

Apply the constant multiple rule $$$\int c f{\left(v \right)}\, dv = c \int f{\left(v \right)}\, dv$$$ with $$$c=\frac{2 g r^{2} \sigma}{9}$$$ and $$$f{\left(v \right)} = v$$$:

$${\color{red}{\int{\frac{2 g r^{2} \sigma v}{9} d v}}} = {\color{red}{\left(\frac{2 g r^{2} \sigma \int{v d v}}{9}\right)}}$$

Apply the power rule $$$\int v^{n}\, dv = \frac{v^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$ with $$$n=1$$$:

$$\frac{2 g r^{2} \sigma {\color{red}{\int{v d v}}}}{9}=\frac{2 g r^{2} \sigma {\color{red}{\frac{v^{1 + 1}}{1 + 1}}}}{9}=\frac{2 g r^{2} \sigma {\color{red}{\left(\frac{v^{2}}{2}\right)}}}{9}$$

Therefore,

$$\int{\frac{2 g r^{2} \sigma v}{9} d v} = \frac{g r^{2} \sigma v^{2}}{9}$$

Add the constant of integration:

$$\int{\frac{2 g r^{2} \sigma v}{9} d v} = \frac{g r^{2} \sigma v^{2}}{9}+C$$

$$$\int \frac{2 g r^{2} \sigma v}{9}\, dv = \frac{g r^{2} \sigma v^{2}}{9} + C$$$A