\[\boxed{\text{885.\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[6 < x < 7\ \ \ \ и\ \ \ \ \ 10 < y < 12\]
\[\textbf{а)}\ x + y\]
\[+ \left| \begin{matrix} 6 < x < 7 \\ 10 < y < 12 \\ \end{matrix} \right.\ \]
\[\text{\ \ \ \ }\overline{16 < x + y < 19}\]
\[\textbf{б)}\ y - x = y + ( - x)\]
\[+ \left| \begin{matrix} - 7 < - x < - 6 \\ 10 < y < 12 \\ \end{matrix} \right.\ \]
\[\text{\ \ \ \ \ }\overline{3 < y - x < 6\ \ }\]
\[\textbf{в)}\ xy\]
\[х\left| \begin{matrix} 6 < x < 7 \\ 10 < y < 12 \\ \end{matrix} \right.\ \]
\[\overline{\ \ \ \ 60 < xy < 84}\]
\[\textbf{г)}\frac{y}{x} = y \cdot \frac{1}{x}\]
\[6 < x < 7\]
\[\frac{1}{7} < \frac{1}{x} < \frac{1}{6}\]
\[х\ \left| \begin{matrix} \frac{1}{7} < \frac{1}{x} < \frac{1}{6} \\ 10 < y < 12 \\ \end{matrix} \right.\ \]
\[\text{\ \ \ }\overline{\frac{10}{7} < \frac{y}{x} < \frac{12}{6}\ \ \ \Longrightarrow 1\frac{3}{7} < \frac{y}{x} < 2}\]