一、一个公式若S_n表示数列{a_n)的前n项和,即S_n=a_1+ a_2+…+a_(n-1)+a_n,则有S_(n-1)=a_1+a_2+…+a_(n-1) (n≥2),于是当n≥2时,a_n=S_n-S_(n-1),而n=1时,a_1=S_1,因此,a_n=(?)．解有关数列题目时,我们常常使用这个公式来实现问题的转化,下面举几个例子加以说明．例1数列{a_n)的前n项和为S_n=3n~2+n +1,则此数列的通项a_n= First, a formula if S_n represents the first n terms of the sequence {a_n), that is S_n=a_1+a_2+...+a_(n-1)+a_n, then S_(n-1)=a_1+a_2+...+a_(n -1) (n≥2), so when n≥2, a_n=S_n-S_(n-1), and n=1, a_1=S_1, therefore, a_n=(?). When solving a series of questions, we often use this formula to achieve the transformation of the problem. Here are a few examples to illustrate. Example 1 The first n items of the sequence {a_n) are S_n=3n~2+n +1, then the general term a_n= of this sequence