Question

PROVE:
-(cos^2 x - sin ^2 x) = 1 - 2cos^2 x

Answer 1

This takes knowledge of identities... specifically, the pythagorean identities... heard of them?

Answer 2

trick..
on the left you have "sin" and "cos"
on the right, there is just a "cos"

so, how can one convert "sin" to "cos"?
what is that identity?