Respuesta :
add 2ab cos C to both sides of the '=':-
c2 + 2ab cos C = a2 + b2
now subtract c2 from both sides:-
2ab cos C = a2 + b2 - c2
c2 + 2ab cos C = a2 + b2
now subtract c2 from both sides:-
2ab cos C = a2 + b2 - c2