x2-y2+2x+2y
=﹙x＋y﹚﹙x－y﹚＋2﹙x＋y﹚
=﹙x＋y﹚﹙x－y＋2﹚
mn-m-n+1
=m﹙n－1﹚－﹙n－1﹚
=﹙m－1﹚﹙n－1﹚