x^2+2y^2-2xy+x-3y+1=0
(x-y)^2+(y-1)^2+x-y=0
(x-y)(x-y+1)+(y-1)^2=0
令y-1=a,y=a+1
（x-a-1）(x-a)+a^2=0
x^2-(2a+1)x+2a^2+a=0
△=（2a+1）^2-4(2a^2+a)=0
4a^2+4a+1-8a^2-4a=0
-4a^2+1=0
a=±1/2
y最大值是1+1/2=3/2
y最小值是1-1/2=1/2