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Question
Express the following as a sum or difference of two trigonometric function:
2 cos 35° cos 75°
Solution
2 cos 35° cos 75°
= cos(35° + 75°) + (35° – 75°)
= cos 110° + cos (– 40)°
= cos 110° + cos 40° ...[∵ cos(– θ) = cos θ]
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