Решебник по алгебре 9 класс Мерзляк Задание 355

\[\boxed{\text{355\ (355).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]

\[y = - x^{2} + 9x + 9\]

\[1)\ x = y\]

\[- x^{2} + 9x + 9 = x\]

\[x^{2} - 8x - 9 = 0\]

\[x_{1} + x_{2} = 8,\ \ x_{1}x_{2} = - 9\]

\[x_{1} = 9,\ \ x_{2} = - 1.\]

\[Ответ:(9;9),\ \ \ ( - 1; - 1).\]

\[2)\ x + y = 25,\ \ y = 25 - x\]

\[- x^{2} + 9x + 9 = 25 - x\]

\[x^{2} - 10x + 16 = 0\]

\[x_{1} + x_{2} = 10,\ \ x_{1} = 8\]

\[x_{1}x_{2} = 16,\ \ x_{2} = 2.\]

\[x = 8,\ \ y = 17;\]

\[x = 2,\ \ y = 23.\]

\[Ответ:(8;17),\ \ \ (2;23).\]