cos3x - cosx =0 Giải giúp pt ạ

TXĐ: $D=R$

$cos3x-cosx=0$

$↔ cos3x=cosx$

$↔ \left[ \begin{array}{l}3x=x+k2\pi\\3x=-x+k2\pi\end{array} \right.$

$↔ \left[ \begin{array}{l}x=k\pi\\x=\dfrac{k\pi}{2}\end{array} \right.$

$↔ x=k\dfrac{\pi}{2}$ $(k∈Z)$