pt
\Leftrightarrow (cosx+1)(cos2x-mcosx)-m(1-cos^2x)=0\Leftrightarrow (cosx+1)(cos2x-mcosx-m(1-cosx))=0
\Leftrightarrow (cosx+1)(cos2x-m)=0
pt da cho co nghiem x\in[0;\frac{2\pi}{3}] khi cos2x=m co nghiem x\in [0;2\pi/3]
Ta co:0\leq x\leq 2\pi/3\Leftrightarrow 0\leq 2x\leq 4\pi/3
khi do -1\leq cos2x\leq 1hay -1\leq m\leq 1
Vay m\in[-1;1]