已知sin=五分之四 a属于 (二分之π ,π),tan(a-b)=二分之一 求tan (a-2b)的值

cosa=±√[1-(sina)^2] = ±3/5∵a∈(π/2 ,π)∴cosa=-3/5tana=sina/cosa = -4/3tan(a-b)=(tana - tanb) / (1 + tanatanb) =1/2解得：tanb=-11/2则：tan2b=2tanb/[1-(tanb)^2] = 44/117tan(a-2b)=-24/7