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1、課時(shí)跟蹤檢測(cè)(三十五) 高考基礎(chǔ)題型得分練1已知數(shù)列an的前n項(xiàng)和為Sn,且對(duì)任意的nN*有anSnn.(1)設(shè)bnan1,求證:數(shù)列bn是等比數(shù)列;(2)設(shè)c1a1且cnanan1(n2),求cn的通項(xiàng)公式(1)證明:由a1S11及a1S1,得a1.又由anSnn及an1Sn1n1,得an1anan11,2an1an1.2(an11)an1,即2bn1bn.數(shù)列bn是首項(xiàng)b1a11,公比為的等比數(shù)列(2)解:由(1)知,2an1an1,2anan11(n2),2an12ananan1(n2),即2cn1cn(n2),又c1a1,2a2a11,a2.c2,即c2c1.數(shù)列cn是首項(xiàng)為,公比為的
2、等比數(shù)列cnn1.2已知數(shù)列an與bn,若a13且對(duì)任意正整數(shù)n滿足an1an2,數(shù)列bn的前n項(xiàng)和Snn2an.(1)求數(shù)列an,bn的通項(xiàng)公式;(2)求數(shù)列的前n項(xiàng)和Tn.解:(1)因?yàn)閷?duì)任意正整數(shù)n滿足an1an2,所以an是公差為2的等差數(shù)列又因?yàn)閍13,所以an2n1.當(dāng)n1時(shí),b1S14;當(dāng)n2時(shí),bnSnSn1(n22n1)(n1)22(n1)12n1,對(duì)b14不成立所以數(shù)列bn的通項(xiàng)公式為bn(2)由(1)知,當(dāng)n1時(shí),T1.當(dāng)n2時(shí),所以Tn.當(dāng)n1時(shí)仍成立,所以Tn.32017山東青島模擬已知數(shù)列an是等差數(shù)列,Sn為an的前n項(xiàng)和,且a1028,S892;數(shù)列bn對(duì)任意n
3、N*,總有b1b2b3bn1bn3n1成立(1)求數(shù)列an,bn的通項(xiàng)公式;(2)記cn,求數(shù)列cn的前n項(xiàng)和Tn.解:(1)設(shè)等差數(shù)列an的公差為d,則a10a19d28,S88a1d92,解得a11,d3,所以an13(n1)3n2.因?yàn)閎1b2b3bn1bn3n1,所以b1b2b3bn13n2(n2),兩式相除,得bn(n2)因?yàn)楫?dāng)n1時(shí),b14適合上式,所以bn(nN*)(2)由(1)知,cn,則Tn,Tn,得Tn2,從而Tn23,即Tn7.4數(shù)列an滿足a11,an12an(nN*),Sn為其前n項(xiàng)和數(shù)列bn為等差數(shù)列,且滿足b1a1,b4S3.(1)求數(shù)列an,bn的通項(xiàng)公式;(2
4、)設(shè)cn,數(shù)列cn的前n項(xiàng)和為T(mén)n,求證:Tn.(1)解:由題意知,an是首項(xiàng)為1,公比為2的等比數(shù)列,ana12n12n1.Sn2n1.設(shè)等差數(shù)列bn的公差為d,則b1a11,b413d7,d2,bn1(n1)22n1.(2)證明:log2a2n2log222n12n1,cn,Tn.nN*,Tn0,數(shù)列Tn是一個(gè)遞增數(shù)列,TnT1.綜上知,Tn(m25m)對(duì)所有的nN*恒成立的整數(shù)m的取值集合(1)證明:依題意,當(dāng)n1時(shí),a29a110100,故10.當(dāng)n2時(shí),an19Sn10,an9Sn110,兩式相減,得an1an9an,即an110an,10,故an為等比數(shù)列,且ana1qn110n(nN*),lg ann.lg an1lg an(n1)n1,即lg an是等差數(shù)列(2)解:由(1)知,Tn333.(3)解:Tn3,當(dāng)n1時(shí),Tn取最小值.依題意有(m25m),解得1m6,故所求整數(shù)m的取值集合為0,1,2,3,4,5