sinA=3/5 cosA=√[1-（3/5)^2]=4/5 (锐角)
tan（A-B）=sin（A-B）/cos(A-B)=(sinAcosB-cosAsinB)/cosAcosB+sinAsinB=-1/2
tanB=2 sinB=2√5/5 cosB=√5/5
cosC=cos（180-A-B）=-cos（A+B）=-[cosAcosB-sinAsinB]=2/25