`~rai~`
\(\cos3x=-\cos\left(x+\dfrac{\pi}{3}\right)\\\Leftrightarrow\cos3x=\cos\left(\pi+x+\dfrac{\pi}{3}\right)\\\Leftrightarrow \cos3x=\cos\left(x+\dfrac{4\pi}{3}\right)\\\Leftrightarrow \left[\begin{array}{I}3x=x+\dfrac{4\pi}{3}+k2\pi\\3x=-x-\dfrac{4\pi}{3}+k2\pi\end{array}\right.\\\Leftrightarrow \left[\begin{array}{I}2x=\dfrac{4\pi}{3}+k2\pi\\4x=-\dfrac{4\pi}{3}+k2\pi\end{array}\right.\\\Leftrightarrow \left[\begin{array}{I}x=\dfrac{2\pi}{3}+k\pi\\x=-\dfrac{\pi}{3}+k\dfrac{\pi}{2}.\end{array}\right.\quad(k\in\mathbb{Z})\\\text{Vậy S=}\left\{\dfrac{2\pi}{3}+k\pi;-\dfrac{\pi}{3}+k\dfrac{\pi}{2}\Big|k\in\mathbb{Z}\right\}.\)
