x²＋3xy+2y²
=(x²＋xy)＋(2xy＋2y²)[把3xy拆成xy＋2xy]
=x(x＋y)＋2y(x＋y)(分别提取公因x和2y)
=(x＋y)(x＋2y)(提取公因x＋y)