1. y = x + 5 có TXĐ là R
\(y=\dfrac{x^2-25}{x-5}\) có TXĐ là \(R\backslash\left\{5\right\}\)
Chọn D.
2. ĐKXĐ \(18-2x\ne0\) \(\Leftrightarrow x\ne9\)
Chọn D
3. ĐKXĐ \(\left\{{}\begin{matrix}x+2\ge0\\x+3\ge0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x\ge-2\\x\ge-3\end{matrix}\right.\) \(\Leftrightarrow x\ge-2\)
Chọn B.
4. ĐKXĐ \(\left\{{}\begin{matrix}4-x^2\ge0\\x^2-x-2\ne0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x^2\le4\\\left(x-2\right)\left(x+1\right)\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\left[{}\begin{matrix}x\ge-2\\x\le2\end{matrix}\right.\\\left[{}\begin{matrix}x\ne2\\x\ne-1\end{matrix}\right.\end{matrix}\right.\) . Chọn câu A
