\[\boxed{\text{499\ (499).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \left\{ \begin{matrix} x \geq 0 \\ y \geq 0 \\ \end{matrix} \right.\ \]
\[\textbf{б)}\ \left\{ \begin{matrix} x \leq 0 \\ y \leq 0. \\ \end{matrix} \right.\ \]