Question

Prove the identity with steps.

Answer 1

Remember that a double angle formula for cosine is
$\cos(2x)=\cos^{2}x-\sin^{2}x$

So
$\cos^{2}(2x)-\sin^{2}(2x)=\cos(4x)$
$\rightarrow \cos^{2}(2x)-\sin^{2}(2x)=\cos(2*2x)$
$\rightarrow \cos^{2}(2x)-\sin^{2}(2x)=\cos^{2}(2x)-\sin^{2}(2x)$