`a)`
`| x^3 - x - 1 | = x^3 + x + 1`
`Với x^3 - x - 1 ≥ 0`
`⇒ x^3 - x - 1 = x^3 + x + 1`
`⇔ 2x = -2`
`⇒ x = -1 (loại)`
`Với x^3 - x - 1 < 0`
`⇒ -x^3 + x + 1 = x^3 + x + 1`
`⇔ -x^3 = x^3`
`⇒ 2x^3 = 0`
`⇒ x = 0 (tm)`
`Vậy x = 0`
`b)`
`| x^4 + x^2 + 1 | = x^2 - x + 1`
`Do x^4 + x^2 ≥ 0`
`⇒ x^4 + x^2 + 1 ≥ 1`
`⇒ x^4 + x^2 + 1 = x^2 - x + 1`
`⇒ x^4 + x = 0`
`⇒ x(x^3 + 1) = 0`
`⇒ \(\left[ \begin{array}{l}x = 0\\x = -1\end{array} \right.\) `
`Vậy S = {0;-1}`
