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Question
Express the following as a sum or difference
sin 5θ sin 4θ
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Solution
sin 5θ sin 4θ
We know
sin A sin B = `1/2`[cos(A – B) – cos(A + B)]
Take A = 5θ, B = 4θ
sin 5θ . sin 4θ = `1/2`[cos(5θ – 4θ) – cos(5θ + 4θ)]
sin 5θ . sin 4θ = `1/2`[cos θ – cos 9θ]
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