cos²15°sin²15°
= 1/4 × 4 × cos²15°sin²15°
= 1/4 × (2cos15°sin15°)²
= 1/4 × (sin30°)²
= 1/4 × 1/4
= 1/16
∵2sinαcosα = sin2α,∴2cos15°sin15° = sin30°