Cho hàm số

B

- Hướng dẫn giải

Chọn B.

$\begin{array}{l}{y}^{\text{'}}={\left({\cot }^2\frac{x}{4}\right)}^{\text{'}}\\ =2\cot \frac{x}{4}{\left(\cot \frac{x}{4}\right)}^{\text{'}}\\ =-\frac{1}{2}\cot \frac{x}{4}\left(1+{\cot }^2\frac{x}{4}\right)\end{array}$

$\begin{array}{l}y\text{'}=0\\ \iff \frac{1}{2}\cot \frac{x}{4}\left(1+{\cot }^2\frac{x}{4}\right)=0\end{array}$

$\begin{array}{l}\iff \cot \frac{x}{4}=0\\ \iff \frac{x}{4}=\frac{\pi }{2}+k\pi \\ \iff x=2\pi +k4\pi ,k\in \mathbb{Z}\end{array}$