cos2 2a- sin2 2a= cos 4a

Let 2a = x We know cos 2x = cos2x - sin2x ...[well known double angle formula] now substitute for x, and you have it!

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Kenneth S. answered • 02/05/18

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Let 2a = x

We know cos 2x = cos²x - sin²x ...[well known double angle formula]

now substitute for x, and you have it!

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