`Q = 5x^(n + 2) + 3x^n + 2x^(n + 2) + 4x^n + x^(n + 2) + x^n`
`= (5x^(n + 2) + 2x^(n + 2) + x^(n + 2)) + (3x^n + 4x^n + x^n)`
`= 8x^(n + 2) + 8x^n`
`= 8x^n(x^2 + 1)`
Mà `Q = 0 ⇔ 8x^n. (x^2 + 1) = 0`
`⇒` \(\left[ \begin{array}{l}8x^n=0\\x^2+1=0\end{array} \right.\)
`⇒` \(\left[ \begin{array}{l}x^n=0\\x^2=-1\end{array} \right.\)
`⇒` \(\left[ \begin{array}{l}x=0\\x ∈ ∅\end{array} \right.\)
`⇒ x = 0`
Vậy `Q = 0` khi `x = 0`