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Question
If A = 30°;
show that:
4 cos A cos (60° - A). cos (60° + A) = cos 3A
Solution
Given that A = 30°
LHS = 4 cos A cos (60° – A ). cos (60° + A)
= 4 cos 30° cos (60° – 30°). cos (60° + 30°)
= 4 cos 30° cos 30° cos 90°
= `4(sqrt3/2)(sqrt3/2) (0)`
= 0
RHS = cos 3A
= cos3(30°)
= cos 90°
=0
LHS = RHS
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