我先把你的题目转化为纯式子,这样看有点麻烦 x/3=y/4=z/5 所以y=4x/3 z=5x/3 则(x+y)/(x-2y+3z)=(x+4x/3)/(x-8x/3+15x/3)=(7/3)/(10/3)=7/10=0.7