Answer: 1/8.
- A 1/8
- B 1/4
- C √3/8
- D 1/16
Correct answer: A. 1/8
Explanation: Using product formula: cos 20° cos 40° cos 80° = (1/4)sin(2<sup>3</sup> × 20°)/sin(20°) ... = 1/8. Standard product identity result.
Unit circle with standard angles 0, 30, 45, 60, 90 degrees; at 45 degrees the point is (cos45, sin45) = (1/sqrt2, 1/sqrt2).
Concept context
Ratios, identities, and applications in triangles