Integral of $$$2 x \sin{\left(3 x \right)}$$$

Find $$$\int 2 x \sin{\left(3 x \right)}\, dx$$$.

Apply the constant multiple rule $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$ with $$$c=2$$$ and $$$f{\left(x \right)} = x \sin{\left(3 x \right)}$$$:

$${\color{red}{\int{2 x \sin{\left(3 x \right)} d x}}} = {\color{red}{\left(2 \int{x \sin{\left(3 x \right)} d x}\right)}}$$

For the integral $$$\int{x \sin{\left(3 x \right)} d x}$$$, use integration by parts $$$\int \operatorname{u} \operatorname{dv} = \operatorname{u}\operatorname{v} - \int \operatorname{v} \operatorname{du}$$$.

Let $$$\operatorname{u}=x$$$ and $$$\operatorname{dv}=\sin{\left(3 x \right)} dx$$$.

Then $$$\operatorname{du}=\left(x\right)^{\prime }dx=1 dx$$$ (steps can be seen ») and $$$\operatorname{v}=\int{\sin{\left(3 x \right)} d x}=- \frac{\cos{\left(3 x \right)}}{3}$$$ (steps can be seen »).

Thus,

$$2 {\color{red}{\int{x \sin{\left(3 x \right)} d x}}}=2 {\color{red}{\left(x \cdot \left(- \frac{\cos{\left(3 x \right)}}{3}\right)-\int{\left(- \frac{\cos{\left(3 x \right)}}{3}\right) \cdot 1 d x}\right)}}=2 {\color{red}{\left(- \frac{x \cos{\left(3 x \right)}}{3} - \int{\left(- \frac{\cos{\left(3 x \right)}}{3}\right)d x}\right)}}$$

Apply the constant multiple rule $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$ with $$$c=- \frac{1}{3}$$$ and $$$f{\left(x \right)} = \cos{\left(3 x \right)}$$$:

$$- \frac{2 x \cos{\left(3 x \right)}}{3} - 2 {\color{red}{\int{\left(- \frac{\cos{\left(3 x \right)}}{3}\right)d x}}} = - \frac{2 x \cos{\left(3 x \right)}}{3} - 2 {\color{red}{\left(- \frac{\int{\cos{\left(3 x \right)} d x}}{3}\right)}}$$

Let $$$u=3 x$$$.

Then $$$du=\left(3 x\right)^{\prime }dx = 3 dx$$$ (steps can be seen »), and we have that $$$dx = \frac{du}{3}$$$.

Thus,

$$- \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 {\color{red}{\int{\cos{\left(3 x \right)} d x}}}}{3} = - \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 {\color{red}{\int{\frac{\cos{\left(u \right)}}{3} d u}}}}{3}$$

Apply the constant multiple rule $$$\int c f{\left(u \right)}\, du = c \int f{\left(u \right)}\, du$$$ with $$$c=\frac{1}{3}$$$ and $$$f{\left(u \right)} = \cos{\left(u \right)}$$$:

$$- \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 {\color{red}{\int{\frac{\cos{\left(u \right)}}{3} d u}}}}{3} = - \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 {\color{red}{\left(\frac{\int{\cos{\left(u \right)} d u}}{3}\right)}}}{3}$$

The integral of the cosine is $$$\int{\cos{\left(u \right)} d u} = \sin{\left(u \right)}$$$:

$$- \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 {\color{red}{\int{\cos{\left(u \right)} d u}}}}{9} = - \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 {\color{red}{\sin{\left(u \right)}}}}{9}$$

Recall that $$$u=3 x$$$:

$$- \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 \sin{\left({\color{red}{u}} \right)}}{9} = - \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 \sin{\left({\color{red}{\left(3 x\right)}} \right)}}{9}$$

Therefore,

$$\int{2 x \sin{\left(3 x \right)} d x} = - \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 \sin{\left(3 x \right)}}{9}$$

Add the constant of integration:

$$\int{2 x \sin{\left(3 x \right)} d x} = - \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 \sin{\left(3 x \right)}}{9}+C$$

$$$\int 2 x \sin{\left(3 x \right)}\, dx = \left(- \frac{2 x \cos{\left(3 x \right)}}{3} + \frac{2 \sin{\left(3 x \right)}}{9}\right) + C$$$A