(x + 1)(x + 2) − (x + 3)(x + 4) = 0
$x^2 + x + 2x + 2 - (x^2 + 3x + 4x + 12) = 0$
$x^2 + 3x + 2 - x^2 - 7x - 12 = 0$
−4x = 10
$x = -\frac{10}{4} = -\frac{5}{2} = -2\frac{1}{2}$
Ответ: $-2\frac{1}{2}$

$10x^2 - (2x - 3)(5x - 1) = 31$
$10x^2 - (10x^2 - 15x - 2x + 3) = 31$
$10x^2 - 10x^2 + 17x - 3 = 31$
17x = 31 + 3
17x = 34
x = 2
Ответ: 2

(x − 2)(x − 3) − (x + 1)(x − 4) = 0
$x^2 - 2x - 3x + 6 - (x^2 + x - 4x - 4) = 0$
$x^2 - 5x + 6 - x^2 + 3x + 4 = 0$
−2x + 10 = 0
−2x = −10
x = 5
Ответ: 5

$12x^2 - (4x - 3)(3x + 1) = -2$
$12x^2 - (12x^2 - 9x + 4x - 3) = -2$
$12x^2 - 12x^2 + 5x + 3 = -2$
5x = −2 − 3
5x = −5
x = −1
Ответ: −1