a_n与S_n的关系问题是高中数学的一个重要知识点,也是有关数列知识的高考题中经常考查的内容之一.学生在运用这方面的知识解题时,常常会进入两个误区:(1)在运用公式a_n=S_n-S_(n-1)时,往往忽略n≥2这个条件;(2)缺乏对等差数列求和公式S_n=(n(a_1+a_n))/2的深度理解. The relationship between a_n and S_n is an important knowledge point in high school mathematics, and it is also one of the contents that are frequently examined in the college entrance examination questions concerning series knowledge. When students use this knowledge to solve problems, they often enter two misunderstandings: (1 ) When using the formula a_n=S_n-S_(n-1), the condition of n≥2 is often ignored; (2) The lack of deep understanding of the sum formula of the arithmetic sequence S_n=(n(a_1+a_n))/2 .