6. C
$\sin x\ne -1\Leftrightarrow x\ne \dfrac{3\pi}{2}+k2\pi$
7. B
$x\ge 0\to D=[0;+\infty)$
8. D
ĐK: $\sin 3x\ne \sin x$
$\Rightarrow 3x\ne x+k2\pi$; $3x\ne \pi-x+k2\pi$
$\Leftrightarrow x\ne k\pi; x\ne \dfrac{\pi}{4}+\dfrac{k\pi}{2}$
9. C
ĐK: $\cos x\ne 0$; $\sin x ne 0$
$\Leftrightarrow x\ne \dfrac{\pi}{2}+k\pi$, $x\ne k\pi$
$\Rightarrow x\ne \dfrac{k\pi}{2}$
10. A
Hàm số luôn xác định.
