Nilai dari 20cos20ºcos40ºcos80ºadalah ⋯

Jawaban: 5/2 Penjelasan: cos a. cos b = ½[cos (a+b) + cos (a-b)] cos 40° cos 20° cos 80°° = 1/2 (cos (40° + 20°) + cos (40° - 20°)) × cos 80° = 1/2 (cos 60° + cos 20°) cos 80° = 1/2 (1/2 + cos 20°) cos 80° = 1/4 cos 80° + 1/2 cos 80° cos 20° = 1/4 cos 80° + 1/4 × 2 cos 80° cos 20° = 1/4 cos 80° + 1/4 (cos 100° + cos 60°) = 1/4 cos 80° + 1/4 cos 100° + 1/4 cos 60° = 1/4 cos 80° + 1/4 (- cos 80°) + 1/4 × 1/2 = 1/8 20 cos 20° cos 40° cos 80° = 20 × 1/8 = 5/2