Tính sin α 4 cos α 4 cos α 2 cos a .
Giải thích
B
\(\sin \frac{\alpha }{4}\cos \frac{\alpha }{4}\cos \frac{\alpha }{2}\cos a\)\( = \frac{1}{2}\sin \frac{\alpha }{2}\cos \frac{\alpha }{2}\cos a\)\( = \frac{1}{4}\sin \alpha \cos a\)\( = \frac{1}{8}\sin 2\alpha \)\( = \frac{1}{8}.\frac{1}{3} = \frac{1}{{24}}\).