>>669
ax^2+2bxy+cy^2+px+qy+r=0
b^2-ac=0
(x,y)=(0,0)
r=0
(x,y)=(1,0)
a+p=0
(x,y)=(0,1)
c+q=0
a=c=1
x^2+2xy+y^2-x-y=0 NG
x^2-2xy+y^2-x-y=0 OK
(1-x)^2-2(1-x)y+y^2-(1-x)-y=0
x^2+2xy+y^2-x-3y=0
a=1, c=4
x^2+4xy+4y^2-x-4y=0 OK
x^2-4xy+4y^2-x-4y=0 OK
(1-x)^2+4(1-x)y+4y^2-(1-x)-4y=0
x^2-4xy+4y^2-x=0
(1-x)^2-4(1-x)y+4y^2-(1-x)-4y=0
x^2+4xy+4y^2-x-8y=0