Tìm x: x^3 + 5x^2 - 4x- 20

* Hướng dẫn giải

$\begin{array}{l}{\mathrm{x}}^3+5{\mathrm{x}}^2-4\mathrm{x}-20=0\\ {\mathrm{x}}^2\left(\mathrm{x}+5\right)-4\left(\mathrm{x}+5\right)=0\\ \left(\mathrm{x}+5\right)\left(\mathrm{x}-2\right)\left(\mathrm{x}+2\right)=0\\ \iff \left[\begin{array}{l}\mathrm{x}=-5\\ \mathrm{x}=2\\ \mathrm{x}=-2\end{array}\right.\end{array}$