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The value of cos 20° cos 40° cos 80° is:
A1/8
B1/4
C√3/8
D1/16
Answer & Solution
Correct answer: A. 1/8
cos θ cos(60° − θ) cos(60° + θ) = (cos 3θ)/4, so with θ = 20° the product is cos 60°/4 = 1/8.
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