=>(tanx-1)(tanx-4)=0
=>tanx=1 hoặc tanx=4
=>Chọn C
\(tan^2x-5tanx+4=0\Rightarrow\left[{}\begin{matrix}tanx=1\\tanx=4\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}\dfrac{1}{tanx}=1\\tanx=4\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}cotx=1\\tanx=4\end{matrix}\right.\)