ГДЗ по алгебре 9 класс Макарычев Задание 695

\[\boxed{\text{695}\text{\ (695)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]

\[\textbf{а)}\ c_{1} = 8,2,\ \ c_{2} = 7,4,\ \ d = c_{2} - c_{1} = - 0,8;\]

\[c_{n} = c_{1} + d(n - 1) = 8,2 - 0,8n + 0,8 = 9 - 0,8n\]

\[9 - 0,8n > 0,\]

\[0,8n < 9\]

\[n < 11,25 \Longrightarrow n = 11.\]

\[S_{11} = \frac{2c_{1} + 10d}{2} \cdot 11 = \frac{16,4 - 8}{2} \cdot 11 = 46,2.\]

\[\textbf{б)}\ c_{1} = - 6,5,\ \ c_{2} = - 6,\ \ d = c_{2} - c_{1} = 0,5,\]

\[c_{n} = c_{1} + d(n - 1) = - 6,5 + 0,5n - 0,5 = 0,5n - 7,\]

\[0,5n - 7 < 0 \Longrightarrow n < 14 \Longrightarrow n = 13,\]

\[S_{13} = \frac{2c_{1} + 12d}{2} \cdot 13 = \left( c_{1} + 6d \right) \cdot 13 = ( - 6,5 + 0,5 \cdot 6) \cdot 13 = - 45,5.\]