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