`(4x^3 + 2x^2 + 1999x) : x = 2005`
`⇔ 4x^2 + 2x + 1999 = 2005`
`⇔ 4x^2 + 2x - 6 = 0`
`⇔ 2(2x^2 + x - 3) = 0`
`⇔ 2(x-1)(2x+3) = 0`
`⇔`\(\left[ \begin{array}{l}x-1=0\\2x+3=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=1\\x=-\dfrac{3}{2}\end{array} \right.\)
Vậy `S = {1,-3/2}`