Đáp án: $ C = \frac{\sqrt[]{3}}{8}$
Giải thích các bước giải:
$ C = cos10.cos50.cos70 = (\frac{1}{2})(cos60 + cos40)cos70 $
$ = (\frac{1}{2})(cos70cos60 + cos70cos40) $
$ = (\frac{1}{2})[(\frac{1}{2})(cos130 + cos10) + (\frac{1}{2})(cos110 + cos30)] $
$ = (\frac{1}{2})[(\frac{1}{2})(cos130 + cos110) + (\frac{1}{2})cos10 + (\frac{1}{2})cos30] = (\frac{1}{2})[cos120cos10 + (\frac{1}{2})cos10 + (\frac{1}{2})cos30] $
$ = (\frac{1}{2})[- (\frac{1}{2})cos10 + (\frac{1}{2})cos10 + (\frac{1}{2})cos30]$
$ = (\frac{1}{4})cos30 = (\frac{1}{4})(\frac{\sqrt[]{3}}{2}) = \frac{\sqrt[]{3}}{8}$
