﹙x²-y²﹚﹙x+y﹚-﹙x-y﹚³
=﹙x-y﹚﹙x+y﹚²-﹙x-y﹚³
=﹙x-y﹚[﹙x+y﹚²-﹙x-y﹚²]
=4xy﹙x-y﹚
﹙x+2y﹚²-2﹙x+2y﹚+1
=﹙x+2y-1﹚²不是﹙x²-y²﹚怎么变成（x-y）的？x²-y²=﹙x+y﹚﹙x-y﹚