a/ \(x^4-4x^3-2x^2-x^3+4x^2+2x-2x^2+8x+4\)
\(=x^2\left(x^2-4x-2\right)-x\left(x^2-4x-2\right)-2\left(x^2-4x-2\right)\)
\(=\left(x^2-x-2\right)\left(x^2-4x-2\right)\)
\(=\left(x+1\right)\left(x-2\right)\left(x^2-4x-2\right)\)
b/ĐK: \(x^2\ne2\)
\(\frac{x^4+4}{x^2-2}=5x\Leftrightarrow x^4+4=5x\left(x^2-2\right)\)
\(\Leftrightarrow x^4-5x^3+10x+4=0\)
\(\Leftrightarrow\left(x+1\right)\left(x-2\right)\left(x^2-4x-2\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=-1\\x=2\\x=2+\sqrt{6}\\x=2-\sqrt{6}\end{matrix}\right.\)
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