Proving Law of Cosines?

Question

How does (1/2)bc(1- cos A) = ((a - b + c)/2) * ((a + b - c)/2)?

Answer 1

Sorry, but I'm not sure I understand your question. Is that an alternate version of the Law of Cosines? It doesn't look like it to me...?

In any case, I found a good proof of the Law of Cosines here:

http://www.themathpage.com/aTrig/law-of-cosines.htm#proof