# Homework Help: Cosine law

1. Nov 8, 2005

### maria curie

hi,
Is anybody who can explain the proof of cosine law here?

thanks,

2. Nov 8, 2005

### Galileo

Take a triangle OAP. Pick your coordinate system such that O is at the origin and A is on the x-axis. Let P have coordinates (x,y) and let |OA|=a, |OP|=b and |AP|=c. Let $\theta$ be the angle between OP and the x-axis.
Express x and y in terms of $\theta$ and compute c using the pythagorean theorem.

3. Nov 8, 2005

### maria curie

ohh thanks a lot.It was easy.

Last edited: Nov 8, 2005
4. Nov 9, 2005

### blackcode

Or you can dram triangel ABC, then you dram the attitude from A then you use pythagore theorem with AB, and AC then you can figure out.