Cho \(\sin \alpha = \dfrac{1}{3}\)và \(\dfrac{\pi }{2} < \alpha < \pi \). Khi đó \(\cos \alpha \) có giá trị là.
A.\(\cos \alpha  =  - \dfrac{2}{3}\).                                                                                   
B.\(\cos \alpha  = \dfrac{{2\sqrt 2 }}{3}\).                          
C.\(\cos \alpha  = \dfrac{8}{9}\).                                                                                      
D.\(\cos \alpha  =  - \dfrac{{2\sqrt 2 }}{3}\).

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