《中学生数学》2004年第三期中孙建斌老师的《构造“代入式”,巧解最值题》一文,给出了条件为x+y=1(或x+y+z=1)的分式函数最值问题的普遍求法,很有意义.遗憾的是文中的解答过程过于繁锁,更重要的是文中没有给出“K”值的确定方法,按文中的解答方法求“K”值是较难的,本文将给出简单解法及“K”值的确定方法,使《构造》一文更趋于完美. 例1 原题:已知x,y∈R+,且x+y=1,求1/x+4/y的最小值. In the third issue of “Middle School Mathematics”, Sun Jianbin’s “Structured Substitution Type”, the most plausible solution to the problem, gives a formula whose condition is x + y = 1 (or x + y + z = 1). The general method of the function of the most value problem is very meaningful. Unfortunately, the solution process in the paper is too cumbersome. What is more important is that the method for determining the value of “K” is not given in the text. The value of “K” is calculated according to the solution in the text. More difficult, this article will give a simple solution and the “K” value determination method, so that the “Structure” article is more perfect. Example 1 The original title: Known x, y ∈ R +, and x + y = 1, seeking The minimum value of 1/x+4/y.