\(\Leftrightarrow\left(m+1\right)-\left(m+1\right)cos2x-2sin2x+2cos2x=0\)
\(\Leftrightarrow2sin2x-\left(m+3\right)cos2x=m+1\)
Phương trình có nghiệm khi:
\(2^2+\left(m+3\right)^2\ge\left(m+1\right)^2\)
\(\Leftrightarrow4m+12\ge0\)
\(\Rightarrow m\ge-3\)