`3x^4 +x^2 -4 =0`
`<=> 3x^3(x-1) + 3x^2(x-1) + 4x^2 -4 =0`
`<=> 3x^3(x-1) +3x^2(x-1) + (2x +2)(2x-2) =0`
`<=> (x-1)(3x^3 + 3x^2) + 4(x+1)(x-1) =0`
`<=> (x-1)(3x^3 + 3x^2 +4x+4) =0`
`<=> (x-1)(x+1)(3x^2+4) =0`
`<=>` \(\left[ \begin{array}{l}x-1 =0\\x+1 =0 \\ 3x^2 +4 =0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=1\\x=-1\end{array} \right.\)
Vậy `S = {1 ; -1}`
