Hàm số $y = \tan \left( {\dfrac{\pi }{2}\cos x} \right)$ xác định khi
$\begin{array}{l} \cos \left( {\dfrac{\pi }{2}\cos x} \right) \ne 0\\ \Leftrightarrow \dfrac{\pi }{2}\cos x \ne \dfrac{\pi }{2} + k\pi \\ \Leftrightarrow \cos x \ne 1 + 2k\\ \left\{ \begin{array}{l} \cos x \ne 1\left( {k = 0} \right)\\ \cos x \ne - 1\left( {k = - 1} \right) \end{array} \right.\\ \Rightarrow \sin x \ne 0 \Leftrightarrow x \ne k\pi \\ \Rightarrow D = \mathbb{R}\backslash \left\{ {k\pi } \right\} \end{array}$
