Ta có: x4 - 8x - 9 = 0
=> x4 - 8x = 9
=> x(x3 - 8) = 9
=> x(x3 - 23) = 9
=> x(x - 2)(x2 + 2x + 22) = 9
=> x(x - 2)(x2 + 2x + 4) = 9
\(\Rightarrow\left[{}\begin{matrix}x=9\\x-2=9\\x^2+2x+4=9\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=9\\x=11\\x^2+2x=5\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=9\\x=11\\x\left(x+2\right)=5\Rightarrow\left[{}\begin{matrix}x=5\\x+2=5\Rightarrow x=3\end{matrix}\right.\end{matrix}\right.\)
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