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本文以部分国内外初中数学竞赛题为例,谈谈如何巧妙构造二次函数求解字母的取值范围·例1(2002年全国初中数学联赛题)设关于x的方程ax2+(a+2)x+9a=0有两个不相等的实数根x1,x2,且x11x2,那么a的取值范围是()(A)-27a25(B)a52(C)a-72(D)-121a0解:因为题设方程有两个不相等
This article takes some of the domestic and foreign junior high school math contest as an example to talk about how to skillfully construct a quadratic function to solve the letter range. Example 1 (2002 national junior high school mathematics league title) Sets the equation about xx2+(a+2)x +9a=0 There are two unequal real roots x1, x2, and x11x2, then the range of a is ()(A)-27a25(B)a52(C)a-72(D)-121a0 Solution: Because there are two inequalities in the equation