tan(x+y)=(tanx+tany)/(1-tanx*tany)=(1/2+1/5)/(1-1/2*1/5)=7/9tan(x+y+z)=(tan(x+y)+tanz)/(1-tan(x+y)*tanz)=(7/9+1/8)/(1-7/9*1/8)=1因为x.y.z都是锐角,且tanx=1\2,tany=1\5,tanz=1\8,那么有:x