Trigonometric Identities Question 96
Question: $ \cos \frac{\pi }{7}\cos \frac{2\pi }{7}\cos \frac{4\pi }{7}= $
[MP PET 1998]
Options:
A) 0
B) $ \frac{1}{2} $
C) $ \frac{1}{4} $
D) $ -\frac{1}{8} $
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Answer:
Correct Answer: D
Solution:
$ \cos \frac{\pi }{7}.\cos \frac{2\pi }{7}.\cos \frac{4\pi }{7}=[ \frac{\sin ( 2^{3}.\frac{\pi }{7} )}{2^{3}\sin ( \frac{\pi }{7} )} ]=\frac{\sin \frac{8\pi }{7}}{8\sin \frac{\pi }{7}}=-\frac{1}{8} $ .
BETA
