What identities do I need to know with secant, cosecant, and cotangent?
- There are two identities with sec, cosec and cot that you need to know and be able to use:
- tan2x + 1 ≡ sec2x
- 1 + cot2x ≡ cosec2x
- These are not really 'new' identities – they can both be derived from sin2x + cos2x ≡ 1
- These identities are not given in the exam formulae booklet – you have to know them.
- If you forget them (or forget where the '+1' goes) you can always derive them from sin2x + cos2x ≡ 1 as shown above.
- Unless told otherwise, you can assume sin2x + cos2x ≡ 1 in any proofs involving sec, cosec or cot on the exam.