已知x-y-3z=0,x+y-z=0,则（x^2 + y^2 - 3z^2 - xy + yz - zx）/(x^2 - 3y^2 + z^2 + xy + yz + zx )=( )

题目

已知x-y-3z=0,x+y-z=0,则（x^2 + y^2 - 3z^2 - xy + yz - zx）/(x^2 - 3y^2 + z^2 + xy + yz + zx )=( )
1
请主要描述一下解题思路、过程,

答案

x-y-3z=0 1)
x+y-z=0 2)
1)+2),得
x=2z
y=-z
所以原式=(4z^2+z^2-3z^2+2x^2-z^2-2z^2)/(4z^2-3z^2+z^2-2z^2-z^2+2z^2)
=(4+1-3+2-1-2)/(4-3+1-2-1+2)
=1/1
=1