$$\displaystyle\int{ \frac{ 1 }{ 1+3 { \cos }^{ 2 } x } }d x$$
$\frac{\arctan(\frac{\tan(x)}{2})}{2}+С$
$\frac{\left(\cos(x)\right)^{2}\frac{\mathrm{d}}{\mathrm{d}x}(\tan(x))}{4\left(\cos(x)\right)^{2}+\left(\sin(x)\right)^{2}}$
