giai pt: cos(x+pi/3)=cos2x

 `cos (x+π/3) = cos2x`

`<=>` \(\left[ \begin{array}{l}x+\dfrac{π}{3} = 2x+k2π\\x+\dfrac{π}{3}=-2x+k2π\end{array} \right.\) 

`<=>` \(\left[ \begin{array}{l}x=\dfrac{π}{3}+k2π\\x=\dfrac{-π}{9}+k \dfrac{2π}{3}\end{array} \right.\) 

Vậy PT có 2 họ nghiệm như trên.