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