很简单：只是你没有想到!
由x+y+z=3x可得x/(x+y+z)=1/3
同理x+y+z=2y可得y/(x+y+z)=1/2
x/(x+y+z)+y/(x+y+z)+z/(x+y+z)=1,所以z/(x+y+z)=1/6