# Prove that #cos(pi-theta)=-costheta# using the compound angle formulae?

##### 1 Answer

Jul 30, 2017

See below

#### Explanation:

There is a rule for the expansion of compound angles for sine, cosine and tangent functions. For the sake of this answer, I'll only go over the cosine compound angle formulae.

Proof for the compound angle formulae can be found here.