$x^2 - 2x - 9 = 0$
$D = 1^2 + 9 = 10$
$x = 1 ± \sqrt{10}$
$x_1 = 1 - \sqrt{10}$
$x_2 = 1 + \sqrt{10}$
Проверка:
$x_1 + x_2 = 1 - \sqrt{10} + 1 + \sqrt{10} = 2$;
$x_1x_2 = (1 - \sqrt{10})(1 + \sqrt{10}) = 1 - 10 = -9$.

$3x^2 - 4x - 4 = 0$
$D = 2^2 + 3 * 4 = 4 + 12 = 16$
$x = \frac{2 ± \sqrt{16}}{3}$
$x_1 = \frac{2 - 4}{3} = -\frac{2}{3}$
$x_2 = \frac{2 + 4}{3} = \frac{6}{3} = 2$
Проверка:
$3(x_1 + x_2) = 3(-\frac{2}{3} + 2) = 3 * \frac{4}{3} = 4$;
$3x_1x_2 = 3 * (-\frac{2}{3}) * 2 = -2 * 2 = -4$.

$2x^2 + 7x - 6 = 0$
$D = 7^2 + 4 * 2 * 6 = 49 + 48 = 97$
$x = \frac{-7 ± \sqrt{97}}{4}$
$x_1 = \frac{-7 - \sqrt{97}}{4}$
$x_2 = \frac{-7 + \sqrt{97}}{4}$
Проверка:
$2(x_1 + x_2) = 2(\frac{-7 - \sqrt{97}}{4} + \frac{-7 + \sqrt{97}}{4}) = 2 * (-\frac{7}{2}) = -7$;
$2x_1x_2 = 2 * \frac{-7 - \sqrt{97}}{4} * \frac{-7 + \sqrt{97}}{4} = -\frac{97 - 49}{8} = -\frac{48}{8} = -6$.

$2x^2 + 9x + 8 = 0$
$D = 9^2 - 4 * 2 * 8 = 81 - 64 = 17$
$x = \frac{-9 ± \sqrt{17}}{4}$
$x_1 = \frac{-9 - \sqrt{17}}{4}$
$x_2 = \frac{-9 + \sqrt{17}}{4}$
Проверка:
$2(x_1 + x_2) = 2(\frac{-9 - \sqrt{17}}{4} + \frac{-9 + \sqrt{17}}{4}) = 2 * (-\frac{9}{2}) = -9$;
$2x_1x_2 = 2 * \frac{-9 - \sqrt{17}}{4} * \frac{-9 + \sqrt{17}}{4} = -\frac{17 - 81}{8} = \frac{64}{8} = 8$.