应该是Sin(x+y)Cosx-Cos(x+y)Sinx=3/5
sin[(x+y)-x]=3/5
siny=3/5
sin²y+cos²y=1
所以cosy=±4/5
tany=siny/cosy=±3/4
tan2y=2tany/(1-tan²y)=(±3/2)/(7/16)
所以tan2y=-24/7或24/7