### When the angles are different. Is One angle Double the Other?

The identities involving the use of the double angle formulae may be useful in cases where one angle is double another so that you can reduce the equation to the same angle then be able to solve it by earlier methods.

Double angle identities

- sin 2A ≡ 2sin A cos A

cos 2A ≡ cos^{2}A – sin^{2}A

or

cos 2A ≡ 2cos^{2}A – 1

or

cos 2A ≡ 1 – 2sin^{2}A

tan 2A ≡ (2 tan A) / (1 – tan^{2}A)