sina=3/5,且a是第二象限角,则cosa=－4/5
tana=sina/cosa=－3/4
tanb=tan[(a＋b)－tana]
=[tan(a＋b)－tana]/[1＋tan(a＋b)tana]
=[1－(－3/4)]/[1－(3/4)]
=7