Cho biết

A

- Hướng dẫn giải

Ta có:

$3\cos \alpha -\sin \alpha =1\iff 3\cos \alpha =\sin \alpha +1\to 9{\cos }^2\alpha ={\left(\sin \alpha +1\right)}^2$

$\iff 9{\cos }^2\alpha ={\sin }^2\alpha +2\sin \alpha +1\iff 9\left(1-{\sin }^2\alpha \right)={\sin }^2\alpha +2\sin \alpha +1$ 

$\iff 10{\sin }^2\alpha +2\sin \alpha -8=0\iff \left[\begin{array}{l}\sin \alpha =-1\\ \sin \alpha =\frac{4}{5}\end{array}\right..$

$\sin \alpha =-1$ : không thỏa mãn vì $0^0<\alpha <{90}^0.$

 $\sin \alpha =\frac{4}{5}\Rightarrow \cos \alpha =\frac{3}{5}\Rightarrow \tan \alpha =\frac{\sin \alpha }{\cos \alpha }=\frac{4}{3}.$ 

Chọn A.