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Câu 2.Pt $\Leftrightarrow 2cos\frac{2x}{3}.(cos\frac{4x}{3}-\frac{1}{2})+2sin^2x+2sin2x=0$$\Leftrightarrow 2cos\frac{2x}{3}(2cos^2\frac{2x}{3}-\frac{3}{2})+2sin^2x+2sin2x=0$$\Leftrightarrow 4cos^2\frac{2x}{3}-3cos\frac{2x}{3}+2sin^2x+2sin2x=0 $$\Leftrightarrow cos2x+2sin^2x+2sin2x=0$$ \Leftrightarrow 1-2sin^2x+2sin^2x+2sin2x=0$$\Leftrightarrow sin2x=-\frac{1}{2}$

Câu 2.Pt $\Leftrightarrow 2cos\frac{2x}{3}.(cos\frac{4x}{3}-\frac{1}{2})+2sin^2x+2sin2x=0$$\Leftrightarrow 2cos\frac{2x}{3}(2cos^2\frac{2x}{3}-\frac{3}{2})+2sin^2x+2sin2x=0$$\Leftrightarrow 4cos^3\frac{2x}{3}-3cos\frac{2x}{3}+2sin^2x+2sin2x=0 $$\Leftrightarrow cos2x+2sin^2x+2sin2x=0$$ \Leftrightarrow 1-2sin^2x+2sin^2x+2sin2x=0$$\Leftrightarrow sin2x=-\frac{1}{2}$