Trigonometry Identities

Pythagorean_trigonometric_identity

Double-angle formulae

Half-angle formulae

During study i found it is harder to solve questions involving double angles and half angles because it has many idenities and they all have to be memorized.But they can be memorized and get used by doing a lot of trigonometry exercises with varies conditions.

Example
Prove that cosx cos2x + sinx sin2x = cosx

when solving this question it is adviseable to choose for LHS because is easier and have more information rather than RHS

Starting from the left hand side:

=cosx cos2x + sinx sin2x

= cos(-x) = cosx

1.we start by identifying any double angles or half angles or phytogorean identites.

2.In this question,we can found two double angles.

3.Next step. we substitute the angles with suitable identities to prove the RHS.