ГДЗ по алгебре 9 класс Макарычев Задание 612

\[\boxed{\text{612}\text{\ (612)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]

\[c_{7} = c_{1} + 6d = 18,5;\ \ \ \ c_{17} = c_{1} + 16d = - 26,5:\]

\[\left\{ \begin{matrix} c_{1} + 6d = 18,5\ \ \ \ \ \ \\ c_{1} + 16d = - 26,5 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} c_{1} = 18,5 - 6d\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ 18,5 - 6d + 16d = - 26,5 \\ \end{matrix} \right.\ \Longrightarrow\]

\[\Longrightarrow \left\{ \begin{matrix} 10d = - 45\ \ \ \ \ \ \ \\ c_{1} = 18,5 - 6d \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} d = - 4,5 \\ c_{1} = 45,5 \\ \end{matrix} \right.\ \]

\[S_{20} = \frac{2c_{1} + d(n - 1)}{2} \cdot n = \frac{2 \cdot 45,5 - 4,5 \cdot (20 - 1)}{2} \cdot 20 =\]

\[= (91 - 85,5) \cdot 10 = 55.\]