cosA=3/5,sinB=5/13,
sinA=4/5,cosB=12/13
sinC=sin(A+B)
=sinAcosB+cosAsinB
=4/5×12/13+3/5×5/13
=63/65
或
cosA=3/5,sinB=5/13,
sinA=4/5,cosB=-12/13
A>45°,如果B是钝角,135°