First slide

Multiple and sub- multiple Angles

Question

The numerical value of $\cos \frac{π}{9} \cos \frac{2 π}{9} \cos \frac{3 π}{9} \cos \frac{4 π}{9}$ is equal to

Moderate

Solution

we have,
$\cos 20^{0} \cos 40^{0} \cos 60^{0} \cos 80^{0} = \frac{1}{4} \frac{1}{2} \cos 3 (20^{0}) = \frac{1}{16} = 0.0625$

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