Sums, Integrals, and Counting

Trigonometric Identities. sin(x+y) = sinx cosy + cosx siny cos(x+y) = cosx cosy - sinx siny. sinx siny = ½ [ cos(x-y) - cos(x+y) ] cosx cosy = ½ [ cos(x-y) + cos(x+y) ] ................
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