Substitute xy+yz+zx=5, xy+y(4-x-y)+x(4-x-y)=5,

= & gtx^2+x(y-4)+(y^2-4y+5)=0，

X is a real number, =>△ = (y-4) 2-4 (y 2-4y+5) ≥ 0,

The solution is 2/3≤y≤2,

That is, the maximum value of y is 2.

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