cosB=5/13
∴sinB=12/13
∠B为锐角
sinA=3/5,那么cosA=4/5或-4/5
∵0°<∠A<180°
0<cosA<1
∴-4/5舍去
cosC=cos(180-A-B)=-cos(A+B)
=sinA·sinB-cosA·cosB
=(3/5)×(12/13)-(4/5)×(5/13)
=(36/65)-(20/65)
=16/65
cosB=5/13
∴sinB=12/13
∠B为锐角
sinA=3/5,那么cosA=4/5或-4/5
∵0°<∠A<180°
0<cosA<1
∴-4/5舍去
cosC=cos(180-A-B)=-cos(A+B)
=sinA·sinB-cosA·cosB
=(3/5)×(12/13)-(4/5)×(5/13)
=(36/65)-(20/65)
=16/65