Integral of $$$x^{3} y$$$ with respect to $$$x$$$

Find $$$\int x^{3} y\, dx$$$.

Apply the constant multiple rule $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$ with $$$c=y$$$ and $$$f{\left(x \right)} = x^{3}$$$:

$${\color{red}{\int{x^{3} y d x}}} = {\color{red}{y \int{x^{3} d x}}}$$

Apply the power rule $$$\int x^{n}\, dx = \frac{x^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$ with $$$n=3$$$:

$$y {\color{red}{\int{x^{3} d x}}}=y {\color{red}{\frac{x^{1 + 3}}{1 + 3}}}=y {\color{red}{\left(\frac{x^{4}}{4}\right)}}$$

Therefore,

$$\int{x^{3} y d x} = \frac{x^{4} y}{4}$$

Add the constant of integration:

$$\int{x^{3} y d x} = \frac{x^{4} y}{4}+C$$

$$$\int x^{3} y\, dx = \frac{x^{4} y}{4} + C$$$A