解x²+xy-2y²=0
(x²－y²)＋(xy－y²)=0
(x＋y)(x－y)＋y(x－y)=0
(x－y)(x＋y＋y)=0
(x－y)(x＋2y)=0
所以x－y=0 x=y
或者x＋2y=0 x=﹣2y