1)x+4х-212)12x-7х+13)x-2ч-1

1) x^2 + 4x - 21 = 0.

Корешки квадратного уравнения вида ax^2 + bx + c = 0 определяются
по формуле: x12 = (-b +- (b^2 - 4 * a * c) / 2 * a.

x12 = (-4 +- (16 - 4 * 1 * (-21)) / 2 * 1 = (-4 +- 10) / 2;

x1 = (-4 - 10) / 2 = -7; x2 = (-4 + 10) / 2 = 3.

2)12x^2 - 7x + 1 = 0.

x12 = (7 +- (49 - 4 * 12 * 1) / 2 * 12 = (7 +- 1) / 2 * 12;

x1 = (7 - 1) / 12 = 1/2; (7 + 1) / 12 = 2/3.

3) x^2 - 2x - 1 = 0.

x12 = (2 +- (4 - 4 * 1 * (-1)) / 2 * 1 = (2 +- 8) / 2;

x1 = (2 - 8) / 2 = 1 - 2; x2 = (2 +8) / 2 = 1 + 2.