The Sum and Difference Formulas - derivations (3 of 4: Trigonometric Identities) (A level)
In this video I derive the sum and difference formulas and give some explanation as to how and why they are used.
part 4: • The Double Angle Formulas - (4 of 4: Trigo...
Also I forgot to mention another useful application of these formulas is modelling. It is often useful to convert a function in the form of e.g. sinAcosB + cosAsinB into the form sin(A+B) which makes it easier to work with in some situations.