Tìm ĐK để hàm số sau có nghĩa: a./ y = 2 cosx / sinx - 1 b./ y = 2 cosx / cosx - 1 c./ y= 2 sinx + 1 / 1 - cosx

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\(a)y=\dfrac{2\cos x}{\sin x-1}\\ĐKXĐ:\sin x-1\ne 0\\\Leftrightarrow \sin x\ne 1\\\Leftrightarrow x\ne \dfrac{\pi}{2}+k2\pi.(k\in\mathbb{Z})\\TXĐ:D=\mathbb{R}\backslash\left\{\dfrac{\pi}{2}+k2\pi\Big|k\in\mathbb{Z}\right\}.\\b)y=\dfrac{2\cos x}{\cos x-1}\\ĐKXĐ:\cos x-1\ne 0\\\Leftrightarrow x\ne k2\pi.(k\in\mathbb{Z})\\TXĐ:D=\mathbb{R}\backslash\{k2\pi|k\in\mathbb{Z}\}.\\c)y=\dfrac{2\sin x+1}{1-\cos x}\\ĐKXĐ:1-\cos x\ne 0\\\Leftrightarrow \cos x\ne 1\\\Leftrightarrow x\ne k2\pi.(k\in\mathbb{Z})\\TXĐ:D=\mathbb{R}\backslash\{k2\pi|k\in\mathbb{Z}\}.\)