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Question
Express the following as a sum or difference
cos 5θ cos 2θ
Solution
cos 5θ cos 2θ
We know
cosA cosB = `1/2`[cos(A + B) + cos(A – B)]
Take A = 5θ, B = 2θ
cos 5θ . cos 2θ = `1/2`[cos(5θ + 2θ) + cos(5θ – 2θ)]
cos 5θ . cos 2θ = `1/2`[cos 7θ + cos 3θ]
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