Integral of x*cos(x) dx

1. Use integration by parts:

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

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

   Then $\operatorname{du}{\left(x \right)} = 1$.

   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 sine is negative cosine:

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

3. Add the constant of integration:

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

The answer is:

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