(2x + 1)(x + 4) - 3x(x + 2) < 0
2x² + x + 8x + 4 - 3x² - 6x < 0
- x² + 3x +4 < 0
x² - 3x - 4 > 0
Решаем уравнение
x² - 3x - 4 = 0
D=(-3)²-4·(-4)=9+16=25=5²
х=(3-5)/2=-1 или х=(3+5)/2=4
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Ответ. (-∞;-1) U (4: +∞)