Apply the law of cosines to the triangle in the accompanying figure to derive the formula for cos (A - B). B . . . . . If a, b, and c are sides of a triangle ABC and if 0 is the angle opposite c, then what does the law of cosines say? O A. 2 = a2 + b2 - ab cos 0 OB. 2 = a2 + b2 + ab cos 0 O C. 2 =a2 + b2 - 2ab cos 0 OD. 2 =a2 + 62 + 2ab cos 0