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Question
Prove that cos 8θ cos 2θ = cos25θ – sin23θ
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Solution
L.H.S = cos 8θ cos 2θ
= cos(5θ + 3θ) cos(5θ – 3θ)
We know cos(A + B) cos(A – B)
= cos2 A – sin2 B
∴ cos(5θ + 3θ) cos(5θ – 3θ) = cos25θ – sin23θ
= R.H.S
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