cos (x+y) + cos (x-y) = 2cos (x) cos (y)

cos (x) cos (y) - sin (x) sin (y) + cos (x) cos (y) + sin (x) sin (y) = 2cos (x) cos (y)

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

2cos (x) cos (y) = 2cos (x) cos (y)

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