Giải thích các bước giải:
$cos^22x=\dfrac{1}{4}\Leftrightarrow\dfrac{1+cos4x}{2}=\dfrac{1}{4}$
$\Rightarrow cos4x=\dfrac{-1}{2}$
$\Rightarrow\left[\begin{array}{nghiempt}x=\dfrac{\pi}{6}+\dfrac{k\pi}{2}\\x=\dfrac{-\pi}{6}+\dfrac{k\pi}{2}\end{array}\right.$