ĐK:
$$\left\{\begin{matrix} \cos 3x\ne 0\\ \cos x\ne 0 \end{matrix}\right.$$
$\Leftrightarrow$ $$\left\{\begin{matrix} x\ne \dfrac{\pi}{6}+\dfrac{k\pi}{3} \\ x\ne \dfrac{\pi}{2}+k\pi \end{matrix}\right.$$
$\Leftrightarrow x\ne\dfrac{\pi}{6}+\dfrac{k\pi}{3}$
$\to D=\mathbb{R}$ \ $\Big\{ \dfrac{\pi}{6}+\dfrac{k\pi}{3}\Big\}$
