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Câu hỏi: Tập nghiệm của phương trình $2\cos 2x+1=0$ là
A. $S=\left\{ \dfrac{\pi }{3}+k2\pi ,-\dfrac{\pi }{3}+k2\pi ,k\in \mathbb{Z} \right\}$.
B. $S=\left\{ \dfrac{2\pi }{3}+k2\pi ,-\dfrac{2\pi }{3}+k2\pi ,k\in \mathbb{Z} \right\}$.
C. $S=\left\{ \dfrac{\pi }{3}+k\pi ,-\dfrac{\pi }{3}+k\pi ,k\in \mathbb{Z} \right\}$.
D. $S=\left\{ \dfrac{\pi }{6}+k\pi ,-\dfrac{\pi }{6}+k\pi ,k\in \mathbb{Z} \right\}$.
A. $S=\left\{ \dfrac{\pi }{3}+k2\pi ,-\dfrac{\pi }{3}+k2\pi ,k\in \mathbb{Z} \right\}$.
B. $S=\left\{ \dfrac{2\pi }{3}+k2\pi ,-\dfrac{2\pi }{3}+k2\pi ,k\in \mathbb{Z} \right\}$.
C. $S=\left\{ \dfrac{\pi }{3}+k\pi ,-\dfrac{\pi }{3}+k\pi ,k\in \mathbb{Z} \right\}$.
D. $S=\left\{ \dfrac{\pi }{6}+k\pi ,-\dfrac{\pi }{6}+k\pi ,k\in \mathbb{Z} \right\}$.
$2\cos 2x+1=0\Leftrightarrow \cos 2x=-\dfrac{1}{2}\Leftrightarrow 2x=\pm \dfrac{2\pi }{3}+k2\pi \Leftrightarrow x=\pm \dfrac{\pi }{3}+k\pi $.
Đáp án C.