已知sina=3/5 a∈(π/2,π) tan(π-B)=1/2,求tan(a-2B) a∈(π/2,π),sina=3/5--->cosa=-4/5--->tana=-3/4 tan(π-B)=1/2--->tanB=-1/2--->tan2B=(-1)/(1-1/4)=-4/3 --->tan(a-2B)=(4/3-3/4)/(1+1)=7/24