`(x^4 - 8x^2 - 9)(x^2 - 16) = 0`
`⇒` `(x^4 + x^2 - 9x^2 - 9)(x^2 - 4^2) = 0`
`⇒` `[x^2.(x^2+1) - 9(x^2+1)](x-4)(x+4) = 0`
`⇒ (x^2 - 9)(x^2 + 1)(x-4)(x+4) = 0`
`⇒ (x-3)(x+3)(x^2 + 1)(x-4)(x+4)=0`
`⇒` \(\left[ \begin{array}{l}x=3\\x=-3\\x=4\\x=-4\end{array} \right.\)
Vậy `S = {±3;±4}`.