Hàm số \(y=\dfrac{2}{\cos x-\cos 3x}\)
ĐKXĐ: \(\cos x-\cos 3x\ne 0\)
\(\Leftrightarrow \cos x\ne \cos3x\)
$\Leftrightarrow x\ne \pm3x+k2\pi$
\(\Leftrightarrow \left\{ \begin{array}{l} x\ne k\pi \\ x\ne k\dfrac{\pi}{2} \end{array} \right .\)
$\Leftrightarrow x\ne k\dfrac{\pi}{2},(k\in\mathbb Z)$.