已知a为锐角,cosa=3/5
sina=4/5==>tana=4/3
tan(a-b)=-1/3 ==>tan(b-a)=1/3
tanb=tan(b-a+a)=[tan(b-a)+tana]/[1-tan(b-a)*tana]
=(1/3+4/3)/(1-1/3*4/3)=3