等差数列知识梳理1.定义:递推公式表达式:或2.等差数列通项公式:==(当d≠0时,Sn是关于n的一次式)3.等差中项(1)如果,,成等差数列,那么叫做与的等差中项.即:4.等差数列的前n项和公式:=(当d≠0时,Sn是关于n的二次式且常数项为0)5.等差数列的证明方法定义法:若或(常数)是等差数列.6.等差数列的性质:(1)、当时,则有变形:(2)、等差数列中各项下标成等差数列,则项也成等差数列。(3)、若等差数列、的前和分别为,则.(4)、若、为等差数列,则为等差数列(5)、若{}是等差数列,则,…也成等差数列等差性质一、选择题1.已知为等差数列,135246105,99aaaaaa,则20a等于()A.-1B.1C.3D.72.设nS是等差数列na的前n项和,已知23a,611a,则7S等于()A.13B.35C.49D.633.等差数列{}na的前n项和为nS,且3S=6,1a=4,则公差d等于1A.1B53C.-2D34.已知na为等差数列,且7a-24a=-1,3a=0,则公差d=A.-2B.-12C.12D.25.若等差数列{}na的前5项和525S,且23a,则7a()A.12B.13C.14D.156.在等差数列na中,284aa,则其前9项的和S9等于()A.18B27C36D97.已知{}na是等差数列,124aa,7828aa,则该数列前10项和10S等于()A.64B.100C.110D.1208.记等差数列{}na的前n项和为nS,若112a,420S,则6S()A.16B.24C.36D.489.等差数列na的前n项和为xS若=则432,3,1Saa()A.12B.10C.8D.610.设等差数列{}na的前n项和为nS,若39S,636S,则789aaa()A.63B.45C.36D.2711.已知等差数列}{na中,12497,1,16aaaa则的值是()A.15B.30C.31D.6412.等差数列{an}的前m项和为30,前2m项和为100,则它的前3m项和为()A.130B.170C.210D.260二、填空题13.已知是等差数列,且则k=.14.已知等差数列na的前n项和为nS,若1221S,则25811aaaa.15.设等差数列na的前n项和为nS,若972S,则249aaa=16.设等差数列na的前n项和为nS,若535aa则95SS17.等差数列na的前n项和为nS,且53655,SS则4a18.已知等差数列的公差是正整数,且a,则前10项的和S=19.设{}与{}是两个等差数列,它们的前项和分别为和,若,那么___________;20.是等差数列的前n项和,(n≥5,),=336,则n的值是.2