Question 942867
trigonometric identities:
cosine of sum, cosine of difference:
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cos(x + y) = cosx cosy - sinx siny
cos(x - y) = cosx cosy + sinx siny
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cos(x + y) + cos(x - y) = cosx cosy - sinx siny + cosx cosy + sinx siny
cos(x + y) + cos(x - y) = cosx cosy + cosx cosy - sinx siny + sinx siny
cos(x + y) + cos(x - y) = cosx cosy + cosx cosy + 0
cos(x + y) + cos(x - y) = 2 cosx cosy
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