Đáp án đúng: C
Giải chi tiết:\(\eqalign{ & C = \cos \left( {{\pi \over 2} + \alpha } \right) + \cos \left( {2\pi - \alpha } \right) + \sin \left( {\pi - \alpha } \right) + \cos \left( {\pi + \alpha } \right) \cr & \,\,\,\,\, = \cos \left( {{\pi \over 2} - \left( { - \alpha } \right)} \right) + \cos \left( {2\pi - \alpha } \right) + \sin \left( {\pi - \alpha } \right) + \cos \left( {\pi - \left( { - \alpha } \right)} \right) \cr & \,\,\,\,\, = \sin \left( { - \alpha } \right)\,\,\,\,\,\,\,\,\,\, + \cos \left( { - \alpha } \right)\,\,\,\,\,\,\,\,\, + \sin \alpha \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - \cos \left( { - \alpha } \right) \cr & \,\,\,\,\, = - \sin \alpha + \cos \alpha + \sin \alpha - \cos \alpha \cr & \,\,\,\,\, = 0 \cr} \)
Chọn: C