\(\Leftrightarrow y=0\) có 3 nghiệm pb
\(x^3-\left(m+2\right)x^2+mx+1=0\)
\(\Leftrightarrow x^3-2x^2+1-\left(x^2-x\right)m=0\)
\(\Leftrightarrow\left(x-1\right)\left(x^2-x-1\right)-\left(x-1\right).mx=0\)
\(\Leftrightarrow\left(x-1\right)\left(x^2-\left(m+1\right)x-1\right)=0\)
\(\Rightarrow x^2-\left(m+1\right)x-1=0\) có 2 nghiệm pb khác 1
\(\Leftrightarrow\left\{{}\begin{matrix}1-\left(m+1\right)-1\ne0\\\Delta=\left(m+1\right)^2+4>0\end{matrix}\right.\)
\(\Rightarrow m\ne-1\)