Lời giải chi tiết:

Ta có

\(\begin{array}{l}\cos \alpha  =  - \cos \left( {\pi  - \alpha } \right)\\ \Rightarrow \cos \dfrac{\pi }{9} =  - \cos \left( {\pi  - \dfrac{\pi }{9}} \right) =  - \cos \left( {\dfrac{{8\pi }}{9}} \right)\\ \Rightarrow \cos \dfrac{\pi }{9} + \cos \dfrac{{8\pi }}{9} = 0\end{array}\)

\(\begin{array}{l} \Rightarrow \cos \dfrac{\pi }{9} + \cos \dfrac{{2\pi }}{9} + ... + \cos \dfrac{{8\pi }}{9}\\ = \left( {\cos \dfrac{\pi }{9} + \cos \dfrac{{8\pi }}{9}} \right) + \left( {\cos \dfrac{{2\pi }}{9} + \cos \dfrac{{7\pi }}{9}} \right) + \left( {\cos \dfrac{{3\pi }}{9} + \cos \dfrac{{6\pi }}{9}} \right) + \left( {\cos \dfrac{{4\pi }}{9} + \cos \dfrac{{5\pi }}{9}} \right)\\ = 0 + 0 + 0 + 0 = 0\end{array}\)

Chọn A