sinx=3/5,则cosx=±4/5
即tanx=±3/4
tan(x-B)=(tanx-tanB)/(1+tanxtanB)
tanx=3/4时,tan(x-B)=(3/4-1/2)/(1+3/8)=2/11
tanx=-3/4时,tan(x-B)=(-3/4-1/2)/(1-3/8)=-2