I did wonder that as I posted. I hoped he'd come back if I hadn't given what he wanted.

There's a proof of the compound angle formulas here:

http://www.mathisfunforum.com/viewtopic … 28#p389628

There is also a proof using Euclidean geometry (strictly only works for angles under 90). Not sure if it's ever been posted … I think maybe it has. If not I'll do it if somebody asks.

edit: found it

http://www.mathisfunforum.com/viewtopic … 88#p410288

Bob

]]>I'm not quite sure what you're after here, but maybe it's this:

So let A+B = P and A-B = Q

Substitute in the above (I've swapped the LHS with the RHS)

Hope that helps,

Bob

]]>I am having trouble understanding the steps of deriving the above mentioned functions. I know how to get their value and also how to derive their value from sine and cosine. But what I want to know is that how did they derive it first time ? How would they think of drawing such a figure ? I would be grateful if you guys provide me with the train of thought which led to the figures and eventually the derivations.]]>