Bạn tham khảo:
cos 2x = - $$\frac{1}{2}$$ $$\\$$ <=> cos 2x = cos $$\frac{2\pi}{3}$$ $$\\$$ <=> 2x = $$\pm$$ $$\frac{2\pi}{3}$$ + k.2$$\pi$$ (K $$\in$$ $$\mathbb{Z}$$) $$\\$$ <=> x = $$\pm$$ $$\frac{\pi}{3}$$ + k.$$\pi$$ (K $$\in$$ $$\mathbb{Z}$$) $$\\$$ Ta có: x $$\in$$ ($$\pi$$;2$$\pi$$) $$\\$$ => +) $$\pi$$ < $$\frac{\pi}{3}$$ + k.$$\pi$$ < 2$$\pi$$ $$\\$$ <=> $$\frac{2}{3}$$ < K < $$\frac{5}{3}$$ $$\\$$ => K = 1 ( do K $$\in$$ $$\mathbb{Z}$$) $$\\$$ +) $$\pi$$ < - $$\frac{\pi}{3}$$ + k.$$\pi$$ < 2$$\pi$$ $$\\$$ <=> $$\frac{4}{3}$$ < K < $$\frac{7}{3}$$ $$\\$$ => K = 2
Vậy Phương trình có 2 nghiệm trên khoảng ($$\pi$$;2$$\pi$$)=> Đáp án D là đáp án đúng
Chúc bạn học tốt