判别式是在研究二次函数中常常要用到的一个解题工具 .由于在具体使用时考虑不周等原因 ,常常产生解集改变的现象 ,惹出了一些是非祸端 .下面分析几例 .例 1　求函数 y =x -1-2x的值域 .错解　移项 ,两边平方 ,化简为关于x的一元二次方程 ,得x2 +2 ( 1-y)x +y2 -1=0 . ①因方程 Discriminant is a problem-solving tool often used in the study of quadratic functions. Due to such factors as inconvenience in the specific use, the phenomenon of solution set change often arises, causing some non-disadvantages. The following analysis of several cases. Example 1 Find the value range of the function y =x -1-2x. The wrong migration term is squared on both sides and reduced to a quadratic equation about x, so x2 +2 (1-y)x +y2 -1=0 . 1 due to the equation