（2a-b）^2+8ab
=4a²－4ab＋b²＋8ab
=4a²＋4ab＋b²
=﹙2a＋b﹚²
6xy^2-9x^2y-y^3
=－y﹙9x²－6xy＋y²﹚
=－y﹙3x－y﹚²