Điều kiện xác định:
$\begin{array}{l}
{\sin ^2}x - {\cos ^2}x \ne 0\\
\Leftrightarrow {\cos ^2}x - {\sin ^2}x \ne 0\\
\Leftrightarrow \cos 2x \ne 0\\
\Leftrightarrow 2x \ne \dfrac{\pi }{2} + k\pi \Leftrightarrow x \ne \dfrac{\pi }{4} + \dfrac{{k\pi }}{2}\\
\to D = \mathbb{R}\backslash \left\{ {\dfrac{\pi }{4} + \dfrac{{k\pi }}{2}|k \in \mathbb{Z}} \right\}
\end{array}$