已知sina=3/5,a∈(π/2,π),tan(π-b)=1/2,则tan(a-2b)=
tan(π-b)＝-tan(b)=1/2
tan(b)=-1/2
tan(2b)=2*tan(b)/(1-tanb^2)=-4/3
tan(-2b)=4/3
sina=3/5
tan(a)=-3/4
tan(a-2b)=[tan(a)-tan(2b)]/[1+tan(a)*tan(2b)]=7/24