-2x平方-6x+3=
=-2(x²＋3x－(3/2))
=-2(x²＋2·x·(3/2)＋(3/2)²－(3/2)²－(3/2))
=-2(x＋(3/2))²＋(15/2).
∴当x=-(3/2)时,原题目的“二次三项式”的值取得最大值,为15/2.