Is There a Straightforward Proof for the Cosine Rule?

[atwarwithmaths]

Hi Guys/Girls,

I have a maths exam tomorrow and there is a chance I could be asked to prove the cosine rule or sine rule.

Well I have a simple proof of the sine rule but cannot find a simple one for the cosine. They all seem very advanced.

Would anybody have a straightforward proof for the cosine rule?

Thanks,
atwarwithmaths

 

[Diffy]

Wikipedia lists some proofs...
http://en.wikipedia.org/wiki/Cosine_rule

 

[atwarwithmaths]

Thats perfect thanks.. Stupid me, I turn to wikipedia for everything else.!