2x^2 +y^2 +4x +8y +18 = 0
2(x^2 +2x+1)+(y^2+8y+16)=0
2(x+1)^2+(y+4)^2=0
两个平方和等于0
所以x=-1,y=-4
X+Y=-5
2x^2+y^2+4x+8y+18=0,求x+y
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