Trigonometryhard · Past Paper
What is the value of cos 20° cos 40° cos 80°?
A1/2
B1/4
C1/8
D1/16
✓ Correct Answer: C — 1/8
Using the product formula cos A cos 2A cos 4A = sin(2^n A) / (2^n sin A), we get sin 160 / (8 sin 20) = sin 20 / (8 sin 20) = 1/8.
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