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posted by - Sunday, November 8, 2009 at 10:29am.

Prove that: 2sinxsiny = cos(x-y) - cos(x+y)

Rewrite the right side, using cos (x + y) = cos x cos y - sin x sin y cos (x - y) = cos x cos y + sin x sin y You should be familiar with those two identities.

kagr

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