x/4=y/4=z/5,则x=y,z=5y/4，然后把x,z都用y 来替代则（x+y）/(x-2y+3z)=2y/(y-2y+3*5y/4)=2y/(4分之11y)=8/11
8/11，如题可知：y=x，z=4/5X，带入等式可等（x-2y+3z）/(x+y)=8/11
5X=5Y=4Z Z=1.25Y-y+3z/2y=(3.75-1)y/2y 2.75/2=1.375
