﹙a－b﹚²﹙x－2y﹚＋﹙b－a﹚﹙2y－x﹚
=﹙a－b﹚²﹙x－2y﹚＋﹙a-b﹚﹙x-2y﹚
=(a-b)(x-2y)(a-b+1)