4x^2+y^2+4xy+12x-6y+9=0
(2x+y)^2+6(2x+y)+9-12y=0
(2x+y+3)^2=12y
同时：
4x^2+y^2+4xy+12x-6y+9=0
(2x-y)^2+6(2x-y)+9+8xy=0
(2x-y+3)^2=-8xy