化归:是指把待解决的问题,通过某种转化过程,归结到一类已经解决或者比较容易解决的问题中去,最终求得原问题之解答的一种手段和方法.等差数列和等比数列是两种重要的数学模型,有着广泛的应用.对一些既不等差又不等比的特殊数列的问题,经常要转化成等差或等比数列,进而寻得问题的解决,这种思维的特点其实就是典型的化归.一、求通项公式中的化归 Return: refers to the problem to be solved, through a certain transformation process, attributed to a class of problems that have been solved or relatively easy to solve, and finally a means and method to obtain the answer to the original problem. Equal number sequences are two important mathematical models and have a wide range of applications. The problem of some special sequences that are neither equal in arithmetic nor equal in number is often converted into arithmetic or arithmetic progressions to solve problems. The characteristics of this kind of thinking are in fact typical transformations.