Điều kiện xác định:
$\begin{array}{l} \cos \left( {2x - \dfrac{x}{3}} \right) \ne 0\\ \Leftrightarrow 2x - \dfrac{x}{3} \ne \dfrac{\pi }{2} + k\pi \\ \Leftrightarrow \dfrac{{5x}}{3} \ne \dfrac{\pi }{2} + k\pi \\ \Leftrightarrow x \ne \dfrac{{3\pi }}{10} + \dfrac{{3k\pi }}{5}\left( {k \in \mathbb{Z}} \right)\\ D = \mathbb{R}\backslash \left\{ {\dfrac{{3\pi }}{10} + \dfrac{{3k\pi }}{2}|k \in \mathbb{Z}} \right\} \end{array}$
