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  4. cos 20° ⋅ cos 40° ⋅ cos 80°=

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Q. $\cos 20^{\circ} \cdot \cos 40^{\circ} \cdot \cos 80^{\circ}=$

Solution:

$\cos \theta \cdot \cos \left(60^{\circ}+\theta\right) \cdot \cos \left(60^{\circ}-\theta\right)=\frac{1}{4} \cos 3 \theta$