设x=2，y=3，z=5（2-3+3*5）/（3*2+2*3）=7/6
y=3x/2;z=5x/2;(x-y+3z) / (3x+2y) =(x-3x/2+3*5x/2)/(3x+2*3x/2)=7/6
7/6
