Proving Identities – Half angles based on the Double Angle formulae
Some identities work with half angles which are based on the double angle identities.
Double angle identities
- sin 2A ≡ 2sin A cos A
cos 2A ≡ cos2A – sin2A
or
cos 2A ≡ 2cos2A – 1
or
cos 2A ≡ 1 – 2sin2A
tan 2A ≡ (2 tan A) / (1 – tan2A)