\[\boxed{\text{1073.\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[\textbf{а)}\ y = x² + 2x\]
\[x - любое\ число.\]
\[\textbf{б)}\ y = \frac{x - 1}{1 + x}\]
\[1 + x \neq 0\]
\[x \neq - 1.\]
\[\textbf{в)}\ y = \sqrt{9 + x}\]
\[9 + x \geq 0\]
\[x \geq - 9\]
\[x \geq - 9.\]
\[\textbf{г)}\ y = \sqrt{3 - x}\]
\[3 - x \geq 0\]
\[- x \geq - 3\]
\[x \leq 3.\]