教材原题1(人教A版高中数学教材必修5第44页例3)已知数列{a_n}的前n项和为S_n=n~2+1/2·n,求这个数列的通项公式.这个数列是等差数列吗?如果是,它的首项与公差分别是什么?改编过程原题是已知S_n求a_n的基本题型.我们知道,对于任何数列{a_n},Sn与a_n有如下关系:a_n= Textbook original title 1 (people teach A version of high school mathematics textbooks compulsory 5 Page 44 Example 3) Known number {a_n} of the first n and S_n = n ~ 2 + 1/2 · n, the general term of this series Is this sequence an arithmetic sequence? If so, what is the first item of it and the tolerance? The original title of the adaptation process is the basic one known as S_n and a_n. We know that for any sequence {a_n}, Sn and a_n has the following relationship: a_n =