已知等差数列{an}求前n项的和Sn,可直接代入求和公式求和,而数列{|an|}前n项的和,则不能直接代入公式求和,那么怎样求数列{|an|}的前n项的和呢?笔者认为,关键是去绝对值符号,转化后再求和,即先分清数列{an}中哪些项是正值,哪些项是负值,分类讨论 Known arithmetic sequence {an} seeking the sum of n and Sn, can be directly substituted into the summation formula summing, and the number of {| an |} n before the sum can not directly into the formula summing, then how to find the number of columns {| an |} The first n terms sum? The author believes that the key is to go absolute symbols, converted and then summation, that is, the first column {an} which is positive, which items are negative, classification discuss