x²+xy-2y²=2(x+2y)x²+(y-2)x-2y²-4y=0x²+(y-2)x-2y(y+2)=0(x+2y)(x-y-2)=0解得x=-2y舍或x=y+2又xy=3解得x=-1，y=-3或x=3,y=1所以x+y=-4或x+y=4
