The Cosine Rule

[Mobyseven]

Hey all.

I'll keep it short and sweet - does anybody have a link to a good explanation of the cosine rule, or can somebody post a proof of it here? I've done a search but the pages I found I've had difficulty following the reasoning.

My textbook has just taught me what the rule is, but -somewhat patronisingly - says that the proof is too complex. Being someon who hates being told what I can and cannot understand, I'm now trying to find the proof myself.

Stupid textbook.

Thanks!

[andyandy]

have you looked at either,

http://www.acts.tinet.ie/cosinerule_673.html

or good old wiki (it's under a search of cosine law rather than rule)

http://en.wikipedia.org/wiki/Law_of_cosines

wiki provides a number of proofs.....

[Mobyseven]

I'd already been to the first link and been completely confused, but when I went back there just then I realised where I had gone wrong (I had thought, for some strange reason, that the two columns were related).

Makes sense, thanks.