Tích số cos 10 ∘ . cos 30 ∘ . cos 50 ∘ . cos 70 ∘ bằng

Chọn C

\[\cos 10^\circ .\cos 30^\circ .\cos 50^\circ .\cos 70^\circ  = \cos 10^\circ .\cos 30^\circ .\frac{1}{2}\left( {\cos {{120}^{\rm{o}}} + \cos {{20}^{\rm{o}}}} \right)\]\[ = \frac{{\sqrt 3 }}{4}\left( { - \frac{{\cos 10^\circ }}{2} + \frac{{\cos 30^\circ  + \cos 10^\circ }}{2}} \right)\]\[ = \frac{{\sqrt 3 }}{4}.\frac{1}{4} = \frac{{\sqrt 3 }}{{16}}\].