x^2+xy+y^2-3x-3y+1=0
x^2+(y-3)x+(y^2-3y+1)=0
判别式(y-3)^2-4(y^2-3y+1)>=0
-3y^2+6y+5>=0
y^2-2y-5/3<=0
(y-1)^2-8/3<=0
y有无数个解,题目有问题