I had seen a solution elsewhere to this problem but get stuck on understanding the steps-
please help!
Answer
$\cos(2\cdot 2A)= \cos^2 2A-\sin^2 2A=(\cos^2 A- \sin^2 A)^2-(2\sin A \cos A)^2=\cos^4 A -2\sin^2 A \cos^2 A + \sin^4 A-4 \sin^2 A \cos^2 A$
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