数列{a_n}的第n项a_n与其前n项和S_n常常出现在同一个问题中,特别是在综合题的已知条件中,它们常常“相聚”在一个等式里,表现得很活跃,使得有关数列问题具有一定的综合性和趣味性,探索求解的思路也就显得灵活多样、丰富多彩.一、等差、等比数列的a_n与S_n关系及其特点 The n-th term a_n of the sequence {a_n} often appears in the same problem as its first n-term and S_n, especially in the known conditions of the synthesis, which often “converge” in an equation Active, so that the number of issues related to a certain number of comprehensive and interesting, to explore the idea of ​​the solution also appears flexible, rich and colorful First, the equi-difference, geometric series a_n and S_n relationship and its characteristics