⇒ max = 1 khi cos2(3x) = 0 ⇔ x = \(\dfrac{\pi}{6}+\dfrac{k\pi}{3}\)
min = -2 khi cos2(3x) = 1 ⇔ \(\left[{}\begin{matrix}cos\left(3x\right)=-1\\cos\left(3x\right)=1\end{matrix}\right.\)⇔\(\left[{}\begin{matrix}x=\dfrac{\pi}{3}+\dfrac{k2\pi}{3}\\x=\dfrac{k2\pi}{3}\end{matrix}\right.\)