Prove the cofunction identity using the Addition and Subtraction Formulas.

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

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

then:

cos(x+y)*cos(x-y)=[cos(x)cos(y)-sin(x)sin(y)]*[cos(x)cos(y)+sin(x)sin(y)]

=[cos(x)cos(y)]^2-[sin(x)sin(y)]^2

Good Luck !!

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