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3.Trigonometrical Ratios, Functions and Identities
easy
$\cos \frac{\pi }{7}\cos \frac{{2\pi }}{7}\cos \frac{{4\pi }}{7} = $
A
$0$
B
$\frac{1}{2}$
C
$\frac{1}{4}$
D
$ - \frac{1}{8}$
Solution
(d) $\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] $
$= \frac{{\sin \frac{{8\pi }}{7}}}{{8\sin \frac{\pi }{7}}}$
$= – \frac{1}{8}$.
Standard 11
Mathematics
