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idk even where to begin

That should be a hint to use the sum identity for sin

Memorize the identities for cos 2*theta

Though I could be wrong but I don't think this is an identity.

One way to find out is to find a counterexample

try theta=pi/2
does not work

sin(2x)=2*sin(x)*cos(x)

cos(2x)=2cos^2 (x)-1

Maybe the problem has been written incorrectly.

cos(2x)=1-2sin^2 (x)
so what's sin(x)*cos(2x)?