Question

[Tường Nguyễn Thế]

Tìm m để bất phương trình sau đúng \(\forall x\in R\): \(\left(m+1\right)x^2-2\left(m-1\right)x+3m+6\ge0\)

Answer 1

[Nguyễn Việt Lâm]

Hệ điều kiện: \(\left\{{}\begin{matrix}a>0\\\Delta'\le0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}m+1>0\\\left(m-1\right)^2-\left(3m+6\right)\left(m+1\right)\le0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}m>-1\\-2m^2-11m-5\le0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}m>-1\\\left[{}\begin{matrix}m\le-5\\m\ge-\frac{1}{2}\end{matrix}\right.\end{matrix}\right.\) \(\Rightarrow m\ge-\frac{1}{2}\)