TeX source:
\begin{array}{cclcrcl}\quad\mathbb{D} &=& \mathbb{R}\setminus_{\{k\pi+\frac{\pi}{2}; k \in \mathbb{Z}\}} \\\\f(x) &=& -6x^3\tan(x)-18 \\\\\quad u(x) &=& -6x^3 &\Rightarrow &u'(x)&=& -18x^2 \\\quad v(x) &=& \tan(x) &\Rightarrow &v'(x) &=& \dfrac{1}{\cos^2(x)} \\\\f'(x) &=& -18x^2\cdot\tan(x)-6x^3\cdot \dfrac{1}{\cos^2(x)} \\&=& -6x^2\left(3\tan(x)+\dfrac{x}{\cos^2(x)}\right)\end{array}