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Question
Express the following as a product
cos 65° + cos 15°
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Solution
We know sin C – sin D = `2 cos ("C" + "D")/2 * cos ("C" - "D")/2`
Take C = 65°, D = 15°
cos 65° + cos 15° = `2cos((65^circ + 15^circ)/2) * cos((65^circ - 15^circ)/2)`
cos 65° + cos 15° = `2cos(80^circ/2) * cos(50^circ/2)`
cos 65° + cos 15° = 2 cos 40° . cos 25°
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