Answer:
True.
Step-by-step explanation:
(LHS) cos 6∘ cos 42∘ cos 66∘ cos 78∘
1/4 [cos(66∘+6∘) = cos(66∘-6∘)] • [cos(78∘+42∘)+cos(78∘-42∘)]
= 1/4(sin18∘+12)(-12+cos36∘)
[∵cos72∘ = cos(90∘−18∘) = sin18∘]
1/4[(5-√-1)/4+1/2][−1/2+(5-√+1)/4]
[∵sin(18∘ = (5-√-1)/4, and, cos 36∘ = (5-√+1)/4]
1/4(5-√+1)/4(5-√-1)/4 = (5-1)/64
= 4/64 = 1/16 (RHS)
Therefore, the cosine is equal to 1/16.
