How do you use the composite properties to show that an equation is an identity?

Question

With this equation.

cos(θ-180) = -cosθ

Answer 1

Use the identity

cos(x - y) = cosx cosy + sinx siny

cos(θ - 180°) = cosθ cos(180°) + sinθ sin(180°) = cosθ (-1) + sinθ (0) = -cosθ