`x^4-5x^2+4=0`
⇔`x^4-x^2-4x^2+4=0`
⇔`x^2(x^2-1)-4(x^2-1)=0`
⇔`(x^2-4)(x^2-1)=0`
⇔\(\left[ \begin{array}{l}x^2=4=0\\x^2-1=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x^2=4\\x^2=1\end{array} \right.\)
⇔\(\left[ \begin{array}{l}\left[ \begin{array}{l}x=2\\x=-2\end{array} \right.\\\left[ \begin{array}{l}x=1\\x=-1\end{array} \right.\end{array} \right.\)
