x²-y²=3x+3y①x²-xy+y²=27②①式得：(x+y)(x-y)=3(x+y)移项得：(x+y)(x-y)-3(x+y)=(x+y)(x-y-3)=0则有x+y=0或x-y-3=0,即x=-y或x=y+3……③将x=-y代入②有：(-y)²-(-y)y+y²=y²+y&...