\[\boxed{\text{613}\text{\ (613)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[b_{1} = 4,2;\ \ \ b_{10} = b_{1} + 9d = 15,9:\ \ \ \ \]
\[4,2 + 9d = 15,9\]
\[9d = 11,7\]
\[d = 1,3\]
\[S_{15} = \frac{2c_{1} + d(n - 1)}{2} \cdot n = \frac{8,4 + 1,3 \cdot 14}{2} \cdot 15 = 199,5.\]
\[\boxed{\text{613.}\text{\ }\text{ОК\ ГДЗ\ -\ домашка\ на\ 5}}\]
\[\left\{ \begin{matrix} x^{2} - y^{2} = 30 \\ x + y = 5\ \ \ \ \ \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} (x + y)(x - y) = 30 \\ x + y = 5\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} x - y = 6 \\ x + y = 5 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} 2x = 11\ \ \\ y = 5 - x \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} x = 5,5\ \ \ \\ y = - 0,5. \\ \end{matrix} \right.\ \]
\[Ответ:\ \ (5,5;\ - 0,5).\]