The proof presupposes Pythagoras theorem which is a special case of the law of cosines, making it somewhat a circular argument. There is a proof without using Pythagoras theorem from more basic principles of Euclidean geometry.
Thanks for sharing! I posted a short video on deriving the law of cosines, It should apply for all angles (acute, obtuse, reflex, negative). Hope to get your thoughts.
Hi I have a question, why do you subtract equation 1 and 2 together apart from trying to get rid of h^2. How does it make actual conceptual sense, what is it showing exactly?