题数列1,2,3,1,2,3,…的通项公式an =_______,前n项和Sn=________．(分别用一个式子表示) 如果该题用分段函数表示,马上可以写出其通项,难就难在用一个式子表示．参考答案: (?) (?) 下面给出本题的另一解法． [x]表示x的整数部分(即高斯函数),我们来研究这个数列的序号n与项an之间的关系． n:1→2→3→4→5→6→7→8→ 9→…→ an:1→2→3→1→2→3→1→2→ 3→…→把数列｛an｝与n联系起来可变形为 The general formula for the problem sequence 1, 2, 3, 1, 2, 3,... An = _______, the first n terms and Sn = ________. (represented by a formula, respectively) If the question is expressed by a piecewise function, its general term can be written immediately, and it is difficult to use a formula to represent it. Reference answer: (?) (?) Here is another solution to this problem. [x] denotes the integral part of x (ie Gaussian function). Let’s study the relationship between the sequence number n and the term an of this sequence. n:1→2→3→4→5→6→7→8→ 9→...→ an:1→2→3→1→2→3→1→2→ 3→...→Sum of {an} n can be transformed into