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Question
Express the following as the product of sine and cosine.
cos 2A + cos 4A
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Solution
cos 2A + cos 4A = 2 cos`(("2A + 4A")/2) cos (("2A - 4A")/2)` ...`[∵ cos "C" + cos "D" = 2 cos (("C + D")/2) cos (("C - D")/2)]`
= 2 cos `((6"A")/2) cos ((6 - 2"A")/2)`
= 2 cos(3A) cos (-A) ...[∵ cos(-θ) = cos θ]
= 2 cos 3A cos A
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