Cho \(\cos \alpha  =  - \frac{2}{3}\).

a) \(90^\circ  < \alpha  < 180^\circ \).

b) \(\cot \alpha \, < \,0\).                          

c) \(\sin \alpha  =  - \frac{{\sqrt 5 }}{3}\).                               

d) \(\tan \alpha  = \frac{{\sqrt 5 }}{2}\). 

Đáp án đúng là: A

Ta có \[P = \sin 30^\circ \cos 60^\circ  + sin60^\circ cos30^\circ  = \frac{1}{2} \cdot \frac{1}{2} + \frac{{\sqrt 3 }}{2} \cdot \frac{{\sqrt 3 }}{2} = 1\].

Đáp án đúng là: D

\(A = \left( {\tan 1^\circ  \cdot \tan 89^\circ } \right) \cdot \left( {\tan 2^\circ  \cdot \tan 88^\circ } \right) \cdot ... \cdot \left( {\tan 44^\circ  \cdot \tan 46^\circ } \right) \cdot \tan 45^\circ \)

\[ = \left( {\tan 1^\circ  \cdot \cot 1^\circ } \right) \cdot \left( {\tan 2^\circ  \cdot \cot 2^\circ } \right) \cdot ... \cdot \left( {\tan 44^\circ  \cdot \cot 44^\circ } \right) \cdot \tan 45^\circ \]

\( = 1\).