$\cos \frac{\pi }{7}\cos \frac{{2\pi }}{7}\cos \frac{{3\pi }}{7} =$
$\cos \frac{\pi }{7}\cos \frac{{2\pi }}{7}\cos \frac{{3\pi }}{7} =$
- A
$-\frac{1}{8}$
- B
$\frac{1}{16}$
- C
$\frac{1}{8}$
- D
None
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