x²-2xy-y²+2x+y+2=0;
2x²-4xy-2y²+3x+3y+4=0;
2x²-4xy-2y²+4x+2y+4=0;
(3)-(2) → x-y=0,y=x.
x²-2xy-y²+2x+y+2=-2x^2+3x+2=0
x=-1/2,或2
y=x=-1/2,或2