\(\Delta=\left(m+1\right)^2-4\left(-m+1\right)=m^2+6m-3\)
Để pt có 2 nghiệm âm pb
\(\Leftrightarrow\left\{{}\begin{matrix}\Delta=m^2+6m-3>0\\x_1+x_2=m+1< 0\\x_1x_2=-m+1>0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}\left[{}\begin{matrix}m>-3+2\sqrt{3}\\m< -3-2\sqrt{3}\end{matrix}\right.\\m< -1\\m< 1\end{matrix}\right.\)
\(\Rightarrow m< -3-2\sqrt{3}\)