Hàm số

C

- Hướng dẫn giải

Chọn C

$\begin{array}{l}f\text{'}\left(x\right)=\left[\frac{2}{\cos \left(\pi x\right)}\right]\text{'}\\ =2.\left(\cos \left(\pi x\right)\right)\text{'}.\frac{-1}{{\cos }^2\left(\pi x\right)}\\ =2\pi \left[-\sin \left(\pi x\right)\right].\frac{-1}{{\cos }^2\left(\pi x\right)}\\ =2.\pi \frac{\sin \left(\pi x\right)}{{\cos }^2\left(\pi x\right)}\end{array}$

$f\text{'}\left(3\right)=2\pi .\frac{\sin 3\pi }{{\cos }^{2}3\pi }=0$