cosC=cos(180-A-B)=-COS(A+B)=
COSA=3/5,sinA=4/5
cosB=5/13,sinB=12/13(均利用直角三角形求出）
cos(A+B）=cosAcosb-sinAsinB=3/5×5/13-4/5×12/13=3/13-48/65=-33/65
cosC=-cos(A+B）=33/65