\(a,\sqrt{72x}\) xác định \(\Leftrightarrow72x\ge0\Leftrightarrow x\ge0\)
\(b,\dfrac{2x+3}{\sqrt{x^2-4}}\) xác định \(\Leftrightarrow x^2-4>0\Leftrightarrow\left(x-2\right)\left(x+2\right)>0\)
\(\Leftrightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}x-2>0\\x+2>0\end{matrix}\right.\\\left[{}\begin{matrix}x-2< 0\\x+2< 0\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}x>2\\x>-2\end{matrix}\right.\\\left[{}\begin{matrix}x< 2\\x< -2\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x>2\\x< -2\end{matrix}\right.\)
\(c,\sqrt{\left(2x+1\right)\left(x+2\right)}\) xác định \(\Leftrightarrow\left(2x+1\right)\left(x+2\right)\ge0\)
\(\Leftrightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}2x+1\ge0\\x+2\ge0\end{matrix}\right.\\\left[{}\begin{matrix}2x+1\le0\\x+2\le0\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}x\ge-\dfrac{1}{2}\\x\ge-2\end{matrix}\right.\\\left[{}\begin{matrix}x\le-\dfrac{1}{2}\\x\le-2\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x\ge-\dfrac{1}{2}\\x\le-2\end{matrix}\right.\)
\(d,3-\sqrt{16x^2-1}\) xác định \(\Leftrightarrow16x^2-1\ge0\Leftrightarrow\left(4x-1\right)\left(4x+1\right)\ge0\)
\(\Leftrightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}4x-1\ge0\\4x+1\ge0\end{matrix}\right.\\\left[{}\begin{matrix}4x-1\le0\\4x+1\le0\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}x\ge\dfrac{1}{4}\\x\ge-\dfrac{1}{4}\end{matrix}\right.\\\left[{}\begin{matrix}x\le\dfrac{1}{4}\\x\le-\dfrac{1}{4}\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x\ge\dfrac{1}{4}\\x\le-\dfrac{1}{4}\end{matrix}\right.\)
\(e,\sqrt{\dfrac{3+x}{4-x}}\) xác định \(\Leftrightarrow\left[{}\begin{matrix}3+x\ge0\\4-x>0\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x\ge-3\\x>4\end{matrix}\right.\) \(\Leftrightarrow x>4\)
