Prove the identity:

(hint: use the laws of cosine)

=

cos Ab² + c² - a²

..a.........2abc

Using the above fact, now prove:

(Hint: what wouldcos Bandcos Clook like?)....................b.........c

cos A+cos B+cos C=a² + b² + c²

..a.......b.......c........2abc

thanks in advance.