Prove the identity:
(hint: use the laws of cosine)
cos A = b² + c² - a²
..a.........2abc
Using the above fact, now prove:
(Hint: what would cos B and cos C look like?)
....................b.........c
cos A + cos B + cos C = a² + b² + c²
..a.......b.......c........2abc
thanks in advance.