因为（x-y)^2=x^2-2xy+y^2=6-6=0,所以x-y=0.
(x-y)²=x²+y²-2xy=6-2*3=0x-y=0不用分类讨论
x^2+y^2=6,xy=3x^2+y^2-2xy=6-2*3=0(x-y)^2=0x-y=0
(x-y)^2=x^2+y^2-2xy=6-6=0x-y=0
