高一数学备课组数列通项数列通项一、常用数列一、常用数列通项通项11,,22,,33,,44,……,……11,,11,,33,,55,,77,,99,……,……33,,55,,77,,99,,1111,……,……22,,44,,66,,88,,1010,……,……00,,22,,44,,66,,88,……,……22,,44,,88,,1616,,3232,……,……11,,44,,99,,1616,,2525,……,……11,-,-11,,11,-,-11,,11,……,……--11,,11,-,-11,,11,-,-11,……,……,51,41,31,21an=nan=2n-1an=2n+1an=2nan=2(n-1)an=2nan=n2an=(-1)n-1an=(-1)nnan1数列数列通项通项99,,9999,,999999,,99999999,,9999999999,,…………22,,2222,,222222,,22222222,,2222222222,,…………11,,2222,,333333,,44444444,,5555555555,,…………22,,33,,1010,,1515,,2626,,3535,…,………,871,651,431,211222,1135,924,715,58,1二、观察法求通项:nnann2)12()1(12112)2()1(nnnann110nna)110(92nna)110(9nnna12)1(nnna三、特殊数列的通项:等差数列:______________________________________等比数列:_______________________________________此法的前提:_______________________________________an=a1+(n-1)dan=am+(n-m)dan=amqn-man=a1qn-1(a1、q≠0)是否能判断此数列是等差数列还是等比数列公式法求通项:特征:______________;公式:______________说明:1)单由Sn-Sn-1=an求an,则有n_;2)由Sn-Sn-1=an求an,若n=1时,由an有________,则an=______________3)由Sn-Sn-1=an求an,若n=1时,由an有________,则an=__________________已知Sn,求an2111nSSnSannn≥2a1=S1Sn-Sn-1a1≠S12111nSSnSnn1、已知数列{an}的前n项和为Sn=3n2+2n,求an解:当n≥2时,an=Sn-Sn-1=6n-1当n=1时,a1=S1=5又由an=6n-1得a1=52、已知数列{an}的前n项和为Sn=3n+1,求an解:当n≥2时,an=Sn-Sn-1=3n-3n-1=3n-1(3-1)=2×3n-1当n=1时,a1=S1=4故an=232141nnn故an=6n-1又由an=2×3n-1得2×31-1=2≠a1应用定义解决问题:例:已知数列{an}中,a1=2,an-an-1=2,(n≥2)求an变1:已知数列{an}中,a1=2,an-an-1=n,(n≥2)求an变2:已知数列{an}中,a1=2,an-an-1=2n,(n≥2)求an归纳:在数列{an}中,已知a1,an-an-1=f(n),(n≥2)(其中f(n)可求和)求an变:已知数列{an}中,a1=2,an-2an-1=2,(n≥2)求an变:已知数列{an}中,a1=2,an-2an-1=2n,(n≥2)求an