Nilai 4 cos 20° cos 40° cos 80°

4 cos 20° cos 40° cos 80°
= 2 (cos (20° + 40°) + cos (20° – 40°)) cos 80°
= 2 (cos 60° + cos (-20°)) cos 80°
= 2 (1/2 cos 80° + cos 80° cos 20°)
= 2 (1/2 cos 80° + 1/2 (cos (80° + 20°) + cos (80° – 20°)))
= 2 (1/2 cos 80° + 1/2 cos 100° + 1/2 cos 60°)
= cos 80° + cos 100° + cos 60°
= cos 80° + cos (180° – 80°) + 1/2
= cos 80° – cos 80° + 1/2
= 1/2