$$y=\tan\ x+\frac{1}{3}\tan^{3}x$$
$y=-\frac{-\sin(3x)-3\sin(x)}{6\left(\cos(x)\right)^{3}}$
$\nexists n_{1}\in \mathrm{Z}\text{ : }x=\pi n_{1}+\frac{\pi }{2}$
$y=\frac{\tan(x)\left(\left(\tan(x)\right)^{2}+3\right)}{3}$
$\nexists n_{1}\in \mathrm{Z}\text{ : }x=\pi n_{1}+\frac{\pi }{2}$
