Cos 20° cos 40° cos 80° =

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3.Trigonometrical Ratios, Functions and Identities

easy

$\cos 20^\circ \cos 40^\circ \cos 80^\circ = $

A

$1/2$

B

$1/4$

C

$1/6$

D

$1/8$

Solution

(d) $\cos {20^o}\cos {40^o}\cos {80^o} = \frac{{\sin {2^3}{{20}^o}}}{{{2^3}\sin {{20}^o}}}$

$ = \frac{{\sin {{160}^o}}}{{8\sin {{20}^o}}} = \frac{1}{8}$.

Standard 11

Mathematics

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