Bài 4 :a) \(y=cot\left(2x-\dfrac{\pi}{4}\right)\) xác định \(\Leftrightarrow2x-\dfrac{\pi}{4}\ne k\pi\left(k\in Z\right)\)\(\Leftrightarrow2x\ne\dfrac{\pi}{4}+k\pi\)\(\Leftrightarrow x\ne\dfrac{\pi}{8}+\dfrac{k\pi}{2}\)b) \(y=\dfrac{1}{3-cos^2x}\) xác định \(\Leftrightarrow\left\{{}\begin{matrix}3-cos^2x\ne0\\-1\le cosx\le1\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}cos^2x\ne3\left(luôn.đúng\right)\\-1\le cosx\le1\end{matrix}\right.\)\(\Leftrightarrow-1\le cosx\le1\)Câu trả lời: Bài 4 :a) \(y=cot\left(2x-\dfrac{\pi}{4}\right)\) xác định \(\Leftrightarrow2x-\dfrac{\pi}{4}\ne k\pi\left(k\in Z\right)\)\(\Leftrightarrow2x\ne\dfrac{\pi}{4}+k\pi\)\(\Leftrightarrow x\ne\dfrac{\pi}{8}+\dfrac{k\pi}{2}\)b) \(y=\dfrac{1}{3-cos^2x}\) xác định \(\Leftrightarrow\left\{{}\begin{matrix}3-cos^2x\ne0\\-1\le cosx\le1\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}cos^2x\ne3\left(luôn.đúng\right)\\-1\le cosx\le1\end{matrix}\right.\)\(\Leftrightarrow-1\le cosx\le1\)