\(\Leftrightarrow\left(m^2+2\right)2cos^2x+4m.sin2x=2m^2+6\)
\(\Leftrightarrow\left(m^2+2\right)\left(cos2x+1\right)+4m.sin2x=2m^2+6\)
\(\Leftrightarrow\left(m^2+2\right)cos2x+4m.sin2x=m^2+4\)
Pt đã cho có nghiệm khi:
\(\left(m^2+2\right)^2+\left(4m\right)^2\ge\left(m^2+4\right)^2\)
\(\Leftrightarrow12m^2\ge12\)
\(\Leftrightarrow\left[{}\begin{matrix}m\ge1\\m\le-1\end{matrix}\right.\)




