Tìm x: x^4 - 8x^2 - 9 = 0

* Hướng dẫn giải

$\begin{array}{l}{\mathrm{x}}^4-8{\mathrm{x}}^2-9=0\iff {\mathrm{x}}^4-9{\mathrm{x}}^2+{\mathrm{x}}^2-9=0\\ \iff {\mathrm{x}}^2.\left({\mathrm{x}}^2-9\right)+\left({\mathrm{x}}^2-9\right)\\ \iff \left({\mathrm{x}}^2+1\right)\left({\mathrm{x}}^2-9\right)=0\\ \Rightarrow {\mathrm{x}}^2-9=0\left(\mathrm{do}{\mathrm{x}}^2+1>0\right)\\ \Rightarrow \left[\begin{array}{l}\mathrm{x}-3=0\\ \mathrm{x}+3=0\end{array}\right.\Rightarrow \left[\begin{array}{l}\mathrm{x}=3\\ \mathrm{x}=-3\end{array}\right.\end{array}$