有些不等式问题 ,如果从正面上去直接探求 ,常常感到繁难 ,甚至一筹莫展 ,不妨转换思维角度 ,从不等式的结构和特点出发 ,巧妙构造与之相关的数学模型 ,使问题转化 ,常可得到简捷、清晰的解法 ,让人有耳目一新的感觉 .一、构造函数例 1　已知函数 f(x) =x2 +bx +c,(b、c为常数 Some inequality problems, if they are directly searched from the front, often feel tedious and even beyond their means, they may wish to change their perspectives, proceed from the structure and characteristics of inequalities, and skillfully construct mathematical models related to them so that the problems can be transformed and often obtained in a simple and clear manner. The solution to the problem is refreshing. A. Constructor Example 1 Known function f(x) = x2 + bx + c, (b, c are constants