三式相加得：
x+y+z=4
2x+y=2
x+(x+y)=2
x+(4-z)=2
x-z=-2
因为
x+2z=3
3z=5
z =5/3
x= - 1/3 
y=2 - 2x=2+2/3=8/3
{x= -1/3
{y=8/3
{z =5/3