x-y+z=0````````````````````1
x+y-5z=o``````````````````2
联立1、2解得：
y=3z, x=2z
(x^2+y^2+z^2)/(2x^2+y^2-z^2)
=[(2z)^2+(3z)^2+z^2]/[2(2z)^2+(3z)^2-z^2]
=14z^2/16z^2
=7/8我算出来好像不是7/8啊