# Deduction in Trigonometric

1. Jun 20, 2007

### Hymne

Anybody that can show how to deduce
cos(a+b) = cos(a)cos(b) - sin(a)sin(b)
sin(a+b) = sin(a)cos(b) + sin(b)cos(a)
From the relations that we get from eulers formula..
Should be really simple but I think that I have got some relations wrong so I need to se the real solution.:uhh:
Thanks!

2. Jun 20, 2007

### neutrino

Which relations have you got wrong?

3. Jun 20, 2007

### morson

Your relations are fine. Use this to verify it:

$$cos(a + b) = Re[cis(a)cis(b)]$$
$$sin(a + b) = Im[cis(a)cis(b)]$$

4. Jun 20, 2007

### Hymne

Hehe it turned out to be that I had it all right I had just made a stupid misstake and was to close to detect it.
Thanks for the help, both of you.