để pt có 2 nghiệm pb âm:
\(\left\{{}\begin{matrix}\Delta>0\\S< 0\\P>0\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}m^2-4>0\\m< 0\\1>0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\left[{}\begin{matrix}m>2\\m< -2\end{matrix}\right.\\m< 0\end{matrix}\right.\Rightarrow m< -2\)