What are the compound angle formulae?
- There are six compound angle formulae (also known as addition formulae), two each for sin, cos and tan:
- For sin the +/- sign on the left-hand side matches the one on the right-hand side
- For cos the +/- sign on the left-hand side is opposite to the one on the right-hand side
- For tan the +/- sign on the left-hand side matches the one in the numerator on the right-hand side, and is opposite to the one in the denominator
- You can derive the tan identity by:
- Dividing the numerator and denominator by
- All these formulae are in the formulae booklet – you don't have to memorise them.