TITLE: How to derive the law of cosines without the pythagorean theorem
QUESTION [12 upvotes]: To me, it seems that the Pythagorean theorem is a special case of the law of cosines. However, all derivations that I can find seem to use the Pythagorean theorem in the derivation. Are there any simple (preferably geometric) derivations of the law of cosines that don't reference the Pythagorean theorem?

REPLY [16 votes]: [As requested, posting a comment as a full answer.]
Taken from my answer to the question "What is the most elegant proof of the Pythagorean Theorem?" ...