`~rai~`
\(y=\sqrt{\dfrac{20+19\cos18x}{1-\sin x}}\\ĐKXĐ:\begin{cases}\dfrac{20+19\cos18x}{1-\sin x}\ge 0\\\sin x\ne 1\end{cases}\quad(1)\\\text{Ta có:}\\+)-1\le\cos18x\le1\\\Leftrightarrow -19\le19\cos18x\le19\\\Leftrightarrow 1\le20+19\cos18x\le39\\\Leftrightarrow 20+19\cos18x>0\quad\forall x.\\+)\sin x\le 1\\\Leftrightarrow 1-\sin x\ge 0\quad\forall x.\\\Rightarrow \dfrac{20+19\cos18x}{1-\sin x}>0\quad\forall x\quad nên\\(1)\Leftrightarrow \sin x\ne 1\\\Leftrightarrow x\ne\dfrac{\pi}{2}+k2\pi.(k\in\mathbb{Z})\\TXĐ:D=\mathbb{R}\backslash\left\{\dfrac{\pi}{2}+k2\pi\Big|k\in\mathbb{Z}\right\}.\)