Решебник по алгебре 9 класс Макарычев Задание 989

\[\boxed{\text{989\ (989).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]

\[a_{6} + a_{10} = 5,9\]

\[a_{12} - a_{4} = 2\]

\[a_{25} = ?\]

\[a_{6} + a_{10} = a_{1} + 5d + a_{1} + 9d =\]

\[= 2a_{1} + 14d,\]

\[2a_{1} + 14d = 5,9;\]

\[a_{12} - a_{4} = a_{1} + 11d - a_{1} - 3d =\]

\[= 8d,\]

\[8d = 2,\ \ d = \frac{1}{4} = 0,25;\]

\[2a_{1} + 14d = 5,9\]

\[2a_{1} + 14 \cdot 0,25 = 5,9\]

\[2a_{1} + 3,5 = 5,9\]

\[2a_{1} = 2,4\]

\[a_{1} = 1,2.\]

\[a_{25} = a_{1} + 24d =\]

\[= 1,2 + 24 \cdot 0,25 = 7,2.\]