已知3x-4y-z=0 2x+y-8z=0 求(x^2+y^2+z^2)/(xy+yz+2zx)的值快啊!

由3x-4y-z=0得z=3x-4y ③ 
由2x+y-8z=0得y=8z-2x ④ 
④代入③得 x=3z ⑤ 
y=2z 

将x,y代入
(x²+y²+z²)/(xy+yz+2zx)
=(9z²+4z²+z²)/(6z²+2z²+6z²)
=(14z²)/(14z²)
=1

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