Ta có:
\(sin(\alpha + {\pi \over 6}) - \cos (\alpha - {{2\pi } \over 3})\)
= \(sin\alpha c{\rm{os}}{\pi \over 6} + \cos \alpha \sin {\pi \over 6} - \cos \alpha \cos {{2\pi } \over 3} - \sin \alpha \sin {{2\pi } \over 3}\)
\( = {{\sqrt 3 } \over 2}sin\alpha + {1 \over 2}\cos \alpha + {1 \over 2}\cos \alpha - {{\sqrt 3 } \over 2}\sin \alpha \)
\( = \cos \alpha = {1 \over 3}\)
