\(cos2x=\dfrac{1}{2}\)
\(\Leftrightarrow cos2x=cos\dfrac{\pi}{3}\)
\(\Leftrightarrow\left[{}\begin{matrix}2x=\dfrac{\pi}{3}+k2\pi\\2x=\dfrac{-\pi}{3}+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{6}+k\pi\\x=-\dfrac{\pi}{6}+k\pi\end{matrix}\right.\)
Nghiệm âm lớn nhất : \(x=-\dfrac{\pi}{6}\)