A) 0
B) \[\frac{1}{2}\]
C) \[\frac{1}{4}\]
D) \[-\frac{1}{8}\]
Correct Answer: D
Solution :
\[\cos \frac{\pi }{7}.\cos \frac{2\pi }{7}.\cos \frac{4\pi }{7}=\left[ \frac{\sin \left( {{2}^{3}}.\frac{\pi }{7} \right)}{{{2}^{3}}\sin \left( \frac{\pi }{7} \right)} \right]=\frac{\sin \frac{8\pi }{7}}{8\sin \frac{\pi }{7}}=-\frac{1}{8}\].
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