# Deriving a trig thing

1. Jul 12, 2006

### Pseudo Statistic

Can anyone tell me how to derive the sin(x+y) and cos(x+y) expansions? The ones that are like cos x sin y or sin y cos x + other stuff?
Preferrably, could this be derived with Euler's formula alone? Or something not too geometric? (All those OAs and OBs and XBs and XYs on geometric diagrams confuse me too much to follow)
Thank you.

2. Jul 12, 2006

### mathman

You could use Euler's formula. It is tedious, but straightforward.

3. Jul 12, 2006

### StatusX

Another way is to use the 2X2 rotation matrices R($\theta$), which have R(x)R(y)=R(x+y). This is equivalent to using Euler's formula, only you're working in R^2 instead of the complex numbers.

4. Jul 13, 2006

### VietDao29

There's a short proof at wikipedia, you can view it at the end of this page.
It is, however, not a completed proof, but you can get some ideas about proving it. :)