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