$f(x) = (-2x + 3)(x + 4)$
$f(x) = 0 \Leftrightarrow x = \dfrac{3}{2}; x = -4$
Bảng xét dấu:
$x | $-\infty$ $-4$ $\frac{3}{2}$ $-\infty$
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$f(x)$ | $- $0$ $+$ $0$ $-$
$f(x) > 0 \Leftrightarrow -4 < x < \dfrac{3}{2}$
$f(x) < 0 \Leftrightarrow \left[ \begin{array}{l}x > \dfrac{3}{2}\\x < -4\end{array} \right.$