Question

3. (cos x + cos y)2 + (sin x – sin y)2 = 4 cos x+y/2
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Answer 1

Formula used 1. cosX + cosY = 2cos[ (X + Y) / 2 ] cos[ (X - Y) / 2 ]

2. sinX - sinY = 2cos[ (X + Y) / 2 ] sin[ (X - Y) / 2 ]

3. sin2x + cos2x = 1

Hope it will help you.