已知4x-3y-6y=0,x+2y-7z=0.求x-y+z\x+y+z的值

{4x-3y=6z ①
{x+2y=7z ②
①-②×4得-11y=-22z
y=-2z
把y=-2z代入②得x=3z
所以(x-y+z)/(x+y+z)
=(3z+2z+z)/(3z-2z+z)
=(6z)/(2z)
=3
如果是(x+y+z)/(x-y+z)则最后结果是1/3
x+y+z和x-y+z谁在分母上.