Cho hàm

D

- Hướng dẫn giải

Chọn D

Ta có:  

$\begin{array}{l}{y}^{\text{'}}=-\sin \left(\frac{2\pi }{3}+2x\right).{\left(\frac{2\pi }{3}+2x\right)}^{\text{'}}\\ =-2.\sin \left(\frac{2\pi }{3}+2x\right)\end{array}$

Theo giả thiết Ta có:  $y^{\text{'}}=0\iff \sin \left(\frac{2\pi }{3}+2x\right)=0$

$\begin{array}{l}\iff \frac{2\pi }{3}+2x=k\pi \\ \iff x=-\frac{\pi }{3}+\frac{k\pi }{2}\left(k\in \mathbb{Z}\right)\end{array}$