Tìm x
x4 - 5x2 + 4 = 0
<=> x4 - 4x2 - x2 + 4 = 0
<=> x2(x2 - 4) - (x2 - 4) = 0
<=> (x2 - 4)(x2 - 1) = 0
<=> (x - 2)(x + 2)(x - 1)(x + 1) = 0
<=> \(\left[{}\begin{matrix}x-2=0\\x+2=0\\x-1=0\\x+1=0\end{matrix}\right.\) <=> \(\left[{}\begin{matrix}x=2\\x=-2\\x=1\\x=-1\end{matrix}\right.\)
Vậy x ={2; -2; 1; -1}