题目

已知各项均为正数的等差数列{an}的前n项和为Sn，且a3+a5﹣=0，则S7=（　　） A．8           B．13              C．14             D．20 答案：C【考点】等差数列的通项公式．                                                             【专题】方程思想；综合法；等差数列与等比数列．                                       【分析】由题意可得a4=2，整体代入S7=7a4可得．                                         【解答】解：∵各项均为正数的等差数列{an}的前n项和为Sn，且a3+a5﹣=0，               由等差数列的性质可得a3+a5=2a4，∴2a4﹣=0，解得a4=2，                         ∴S7===7a4=14                                                     故选：C                                                                                              【点评】本题考查等差数列的通项公式和求和公式，属基础题．                     　                                                                                                      

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