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Multiplication And Addition Trigonometry: cos (x + y) cos (x – y)

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cos (x + y) cos (x – y) = …?

SOLUTION

cos (x + y) cos (x – y) = ½ cos (x + y + x – y) + ½ cos (x + y –(x – y))

cos (x + y) cos (x – y) = ½ cos 2x + ½ cos 2y

cos (x + y) cos (x – y) = ½ (cos 2x + cos 2y)

 

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