原式=-x2-y2﹣3xy-﹙x2+y2﹚
=-x2-y2﹣3xy-x2-y2
=-2x2-2y2﹣3xy-﹙x的2次方+y的2次方﹚+[﹣3xy-﹙x的2次方+y的2次方﹚]对采纳吧