﹙2x+y﹚²－﹙x＋2y﹚²
=[﹙2x+y﹚+﹙x＋2y﹚][﹙2x+y﹚－﹙x＋2y﹚]
=（3x+3y）（x-y)
=3（x+y)(x-y)
3ax²－3ay²
=3a（x²－y²）
=3a（x+y)(x-y)