x² + y² - 2xy - 6x - 6y + 7 = 0
(x-y)^2-6x - 6y + 7 = 0
(x-y)^2-6(x +y)+9=2
(x-y-3)^2=2
因此是两条直线
x-y-3=±√2(x-y)^2-6(x +y)+9=2——(x-y-3)^2=2这一步不对啊(x-y)^2-6(x -y)+9=2——(x-y-3)^2=2噢,那没办法做了。好象是用到坐标系旋转。