a,
$y=\dfrac{1}{\sin(2x-1)}$
ĐK: $\sin(2x-1)\ne 0$
$\Leftrightarrow 2x-1\ne k\pi$
$\Leftrightarrow x\ne \dfrac{1}{2}+\dfrac{k\pi}{2}$
$\Rightarrow D=\mathbb{R}$ \ $\{\dfrac{1}{2}+\dfrac{k\pi}{2}\}$
b,
$y=\dfrac{1-\sin x}{\cos x}$
ĐK: $\cos x\ne 0$
$\Leftrightarrow x\ne \dfrac{\pi}{2}+k\pi$
$\Rightarrow D=\mathbb{R}$ \ $\{\dfrac{\pi}{2}+k\pi\}$
c,
$y=\dfrac{1+\cos x}{\sin2x}$
ĐK: $\sin2x\ne 0$
$\Leftrightarrow x\ne \dfrac{k\pi}{2}$
$\Rightarrow D=\mathbb{R}$ \ $\{\dfrac{k\pi}{2}\}$
