The compound angle formulae
This construction illustrates how the formulae for sin(α + β) and cos(α + β) can be derived.[br][br]Drag points B and D.
Can you find diagrams to illustrate the formulae for sin(α - β) and cos(α - β)?
The double angle formulae
The circle has radius 1 and centre at O.[br]Drag the point D and compare[br]- the two expressions for the length DE[br]- the two expression for the length OE.