a)\(x^4-5x^2-6=0 \\<=>(x^2+1)(x^2-6)=0 \\Mà\ x^2+1>0 \\<=>x^2-6=0 \\<=>x^2=6 \\<=>x=\sqrt6\ hoặc\ x=-\sqrt6\)
b) \(x^3-4x^2+5x=0 \\<=>x(x^2-4x+5)=0 \\Mà\ x^2-4x+5>0 \\<=>x=0\)
c)\(x^3-3x^2+4x+8=0 \\<=>(x+1)(x^2-4x+8)=0 \\Mà\ x^2-4x+8>0 \\<=>x+1=0\\<=>x=-1\)
a. \(x^4-5x^2-6=0\)
\(\Leftrightarrow x^4+x^2-6x^2-6=0\)
\(\Leftrightarrow x^2\left(x^2+1\right)-6\left(x^2+1\right)=0\)
\(\Leftrightarrow\left(x^2+1\right)\left(x^2-6\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x^2+1=0\\x^2-6=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}vô\text{ }nghiệm\\x=\pm\sqrt{6}\end{matrix}\right.\)
Vậy: Phương trình có tập nghiệm \(S=\left\{\sqrt{6};-\sqrt{6}\right\}\)
