Integral of 3x*cosx dx

1. Use integration by parts:

   $$\int \operatorname{u} \operatorname{dv} = \operatorname{u}\operatorname{v} - \int \operatorname{v} \operatorname{du}$$

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

   Then $\operatorname{du}{\left(x \right)} = 3$.

   To find $v{\left(x \right)}$:

   1. The integral of cosine is sine:

      $$\int \cos{\left(x \right)}\, dx = \sin{\left(x \right)}$$

   Now evaluate the sub-integral.

2. The integral of a constant times a function is the constant times the integral of the function:

   $$\int 3 \sin{\left(x \right)}\, dx = 3 \int \sin{\left(x \right)}\, dx$$

   1. The integral of sine is negative cosine:

      $$\int \sin{\left(x \right)}\, dx = - \cos{\left(x \right)}$$

   So, the result is: $- 3 \cos{\left(x \right)}$

3. Add the constant of integration:

   $$3 x \sin{\left(x \right)} + 3 \cos{\left(x \right)}+ \mathrm{constant}$$

The answer is:

$$3 x \sin{\left(x \right)} + 3 \cos{\left(x \right)}+ \mathrm{constant}$$