Đáp án đúng: A
Giải chi tiết:\(\eqalign{ & B = \sin \left( {x - {\pi \over 2}} \right) + \cos \left( {x - \pi } \right) - 5\sin \left( {{{11\pi } \over 2} + x} \right) \cr & \,\,\,\,\, = - \sin \left( {{\pi \over 2} - x} \right) + \cos \left( {\pi - x} \right) - 5\sin \left( {x - {\pi \over 2} + 6\pi } \right) \cr & \,\,\,\, = - \cos x\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - \cos x\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - 5\sin \left( {x - {\pi \over 2}} \right) \cr & \,\,\,\, = - \cos x\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - \cos x\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, + 5\sin \left( {{\pi \over 2} - x} \right) \cr & \,\,\,\, = - 2\cos x + 5\cos x = 3\cos x \cr} \)
Chọn: A.