Integral of (cosx+3x^2) dx

1. Integrate term-by-term:

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

      $$\int 3 x^{2}\, dx = 3 \int x^{2}\, dx$$

      1. The integral of $x^{n}$ is $\frac{x^{n + 1}}{n + 1}$ when $n \neq -1$:

         $$\int x^{2}\, dx = \frac{x^{3}}{3}$$

      So, the result is: $x^{3}$

   1. The integral of cosine is sine:

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

   The result is: $x^{3} + \sin{\left(x \right)}$

2. Add the constant of integration:

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

The answer is:

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