是不是x^2-2xy+4y^2-x-2y+1=0?
x^2-2xy+y^2+3y^2-x-2y+1=0
(x-y)^2+3(y^2-y+1/4)-(x-y)+1/4=0
(x-y)^2-(x-y)+1/4+3(y-1/2)^2=0
(x-y-1/2)^2+3(y-1/2)^2=0