求参数的值或取值范围的问题,一般都是寻求题设的充要条件.对于这类问题,若通过直接等价转化的方法寻求,有时比较麻烦,甚至难度还很大.为此,我们可以先考虑题设的必要条件,对参数的取值范围进行初步的限制或收缩,在此基础上再探求充分性成立,往往能化难为易,化繁为简,事半功倍. The problem of seeking the value or the range of the parameter is generally the necessary and sufficient condition for finding the problem.To this kind of problem, if seeking through the method of direct equivalence conversion, sometimes it is more troublesome and even more difficult.For this reason, We can first consider the necessary conditions for setting the initial limits or contraction of the range of parameters, and then explore the adequacy of the establishment based on this, often can make it difficult for the easy, complex and simple, with less.