# Complex exponential help

1. May 1, 2004

### n0_3sc

This is an easy question but can some else show/tell me how to do it:
"use the complex exponential series to express cos(2x) in terms of sin(x)"
I also don't quite understand the 'complex exponential series'.

2. May 1, 2004

### n0_3sc

I mean can someone* show/tell me....

3. May 2, 2004

$$e^{i\theta} = \cos{\theta} + i\sin{\theta}$$

$$e^{i\theta} + e^{-i\theta} = 2\cos{\theta}$$

$$e^{i\theta} - e^{-i\theta} = 2i\sin{\theta}$$