在ABC中,cos a=-12/13,cos B=3/5,求cosC cos a=-12/13sina=5/13,cos B=3/5sinB=4/5 ,cosC=cos(180-A-B)=-cos(A+B)=-cosA*cosB+sinA*sinB=-(-12/13)(3/5)+(5/13)(4/5)=36/65+20/65=56/65