(cos(2x)−sin(2x))(sin(2x)+cos(2x))=cos(4x) I'm trying to prove that the left side equals the right side. I'm just stuck on which double angle formula of cosine to use.
Answer
(cos(2x)−sin(2x))(sin(2x)+cos(2x))=(cos2(2x)−sin2(2x))=cos(4x)
From
cos(a+b)=cosacosb−sinasinb if a=2x,b=2x then
cos(4x)=cos22x−sin22x
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