“正难则反”是数学解题的一种重要方法及技巧,借用反面(即补集思想)减少运算量,真正起到化繁为简、化难为易的目的.在运用时一方面要注意的是真的正难吗?另一方面要注意反面(即命题的否定)是否正确.兹举两例加以分析,希望能引起师生们的注意.例1设集合A={x|x~2+4ax-4a+3=0},集合B={x|x~2+(a-1)x+a2=0},集合C={x|x~2+2ax-2a=0},试确定a的取值范 “It is hard to be the opposite” is an important method and skill of solving mathematical problems, borrowing the negative (that is, complementing thought) to reduce the amount of computation, really play the simplification, simplification is easy .In the use of a On the other hand should pay attention to the negative (that is, the negative proposition) is correct .Take two cases to analyze, hoping to arouse the attention of teachers and students.Example 1 set A = {x (x-2 + 4ax-4a + 3 = 0}, the set B = {x | x ~ 2 + (a-1) x + a2 = 0} 0}, try to determine the value of a range