Answer: −1/8.
- A 1/8
- B −1/8
- C 1/2
- D −1/2
Correct answer: B. −1/8
Explanation: Using cosA·cos2A·cos4A = sin8A/(8 sinA) with A = π/7 gives sin(8π/7)/(8 sin(π/7)) = −1/8.
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