2 câu trả lời
$y=\dfrac{1}{\sin\dfrac{x}{2}-\cos\dfrac{x}{2}}$
ĐK: $\sin\dfrac{x}{2}-\cos\dfrac{x}{2}\ne 0$
$\Leftrightarrow \sqrt2\sin(\dfrac{x}{2}-\dfrac{\pi}{4})\ne 0$
$\Leftrightarrow x\ne \dfrac{\pi}{2}+k2\pi$
$\to D=\mathbb{R}$ \ $\{\dfrac{\pi}{2}+k\pi\}$
