第二章 数列极限总练习题1、求下列数列的极限: (1)limn→∞√n 3+3n n;(2)limn→∞n 5e n;(3)lim n→∞(√n +2−2√n +1+√n).解:(1)当n>3时,n 3<3n ,∴3=√3n n<√n 3+3n n<√2·3n n=3√2n→3(n →∞). 由迫敛性定理可知:lim n→∞√n 3+3n n=3.(2)设a n =n 5e n ,则limn→∞a na n+1=lim n→∞e (nn+1)5=e>1,∴limn→∞n 5e n=0.(3)lim n→∞(√n +2−2√n +1+√n)=lim n→∞[(√n +2−√n +1)−(√n +1−√n)] =lim n→∞[√n+2+√n+1−√n+1+√n]=0.2、证明:(1)lim n→∞n 2q n =0(|q|<1);(2)limn→∞lgn n a=0(a ≥1);(3)lim √n!n=0.证明:(1)当q=0 时,n 2q n =0,lim n→∞n 2q n =0;当0<|q|<1时,令|q|=1p ,则p>1. 设p=1+h ,h>0. 由(1+h)n >13!n(n-1)(n-2)h 3,(n>2) 得0<|n 2q n |<n 2(1+h)n <6h 3·n 2n(n−1)(n−2)=6h 3·1n(1−1n )(1−12)→0(n →∞).由迫敛性定理可知:lim n→∞n 2q n =0 (|q|<1).(2)任给ε>0,则10ε>1,√n n→1(n →∞),故存在N ,当n>N 时,有1<√n n<10ε,取对数后得:0<lgn n<ε,∴limn→∞lgnn=0. 从而当a ≥1时,0<lgn n a ≤lgn n→0(n →∞).由迫敛性定理可知:limn→∞lgn n a=0(a ≥1).(3)任给ε>0,令M=1ε,则limn→∞M nn!=0.又对ε0=1,存在自然数N ,使得当n>N 时,M nn!<1,即1n!<εn , ∴当n>N 时,有0<√n!n <ε,∴lim√n!n=0.3、设lim n→∞a n =a ,证明:(1)limn→∞a 1+a 2+⋯+a nn=a(又问由此等式能否反过来推出lim n→∞a n =a );(2)若a n >0,(n=1,2,…),则lim n→∞√a 1a 2…a n n =a.证:(1)∵lim n→∞a n =a ,∴对任意的ε>0,必存在N 1,使当n>N 1时,|a n -a|<ε,令m=max{|a 1-a|,|a 2-a|,…,|a n -a|},于是n>N 1时, |a 1+a 2+⋯+a nn −a|=|a 1−a+a 2−a+⋯+a n −an|≤1n (|a 1-a|+|a 2-a|+…+|a N 1+1-a|+|a N 1+2-a|+…+|a n -a|)<N 1m n+(n−N 1)nε<N 1m n+ε.又limn→∞N 1m n=0. ∴对已给的ε>0,存在N 2,当n>N 2时,N 1mn<ε.取N=max{N 1,N 2},则当n>N 时,|a 1+a 2+⋯+a nn−a|<2ε,∴limn→∞a 1+a 2+⋯+a nn=a. 此等式反过来不能推出lim n→∞a n =a .例如a n =(-1)n 不收敛,但lim n→∞a 1+a 2+⋯+a nn=0.(2)对任意自然数n ,a n >0,∴当a ≠0. ∴lim n→∞1a n=1a .又11a 1+1a 2+⋯+1a nn=n1a 1+1a 2+⋯+1a n≤√a 1a 2…a n ≤a 1+a 2+⋯+a nn→a (n →∞).由迫敛性定理可知:lim n→∞√a 1a 2…a n n =a.当a=0时,对任给的ε>0,存在N 1,使当n>N 1时,0<a n <ε,于是当n>N 1时,0<√a 1a 2…a n n =√a 1a 2…a N 1n ·√a N 1+1a N 1+2…a n n<√a 1a 2…a N 1n·εn−N 1n<√a 1a 2…a N 1·ε−N 1n·ε,∵lim n→∞√a 1a 2…a N 1·ε−N 1n=1,从而存在N 2,使当n>N 2时,√a 1a 2…a N 1·ε−N 1n<2,故当n>N=max{N 1,N 2}时,必有0<√a 1a 2…a n n <2ε,∴lim n→∞√a 1a 2…a n n=a.4、应用上题的结论证明下列各题: (1)limn→∞1+12+⋯+1nn=0;(2)lim n→∞√a n =1(a>0);(3)lim n→∞√n n=1;(4)lim√n!n=0;(5)lim√n!n=e ;(6)lim n→∞1+√2+⋯+√n nn =1;(7)若limn→∞b n+1b n=a (b n >0),则lim n→∞√b n n =a ;(8)若lim n→∞(a n −a n−1)=d ,则limn→∞a nn=d .证:(1)∵lim n→∞1n =0;∴limn→∞1+12+⋯+1nn =0;(2)设a 1=a, a n =1 (n=2,3…),则lim n→∞a n =1;∴lim n→∞√a n=lim n→∞√a 1a 2…a n n =1.(3)设a 1=1, a n =nn−1 (n=2,3…),则lim n→∞a n =1;∴lim n→∞√n n=lim n→∞√a 1a 2…a n n =1.(4)lim√n!n=lim n→∞√11·12···1n n=limn→∞1n=0.(5)设a n =n nn! (n=1,2…),则a 1=1;lim√n!n=lim n→∞√a n n=lim n→∞√a 2a 1·a 3a 2···a nan−1n=limn→∞a na n−1=lim n→∞(1+1n−1)n−1=e.(6)lim n→∞1+√2+⋯+√n nn =lim n→∞√n n=1. (7)令b 0=1,则lim n→∞√b n n =lim n→∞√b 1b 0·b 2b 1·b3b 2···b nbn−1n=limn→∞b n+1b n=a (b n >0).(8) lim n→∞a nn=lim n→∞[(a 2−a 1)+(a 3−a 2)+⋯+(a n −a n−1)n+a1n ]=lim n→∞(a n −a n−1)=d .5、证明:若{a n }为递增数列,{b n }为递减数列,且lim n→∞(a n −b n )=0,则lim n→∞a n 与lim n→∞b n 都存在且相等.证:∵lim n→∞(a n −b n )=0,∴{a n -b n }有界,不妨设A ≤a n -b n ≤B ,A,B 为常数. ∵{a n }递增,{b n }递减,∴a n ≤B+b n ≤B+b 1,b n ≥a n -B ≥a 1-B. ∴{a n }{b n }单调有界 ∴{a n }{b n }都有极限. 而lim n→∞(a n −b n )= lim n→∞a n −lim n→∞b n =0,∴lim n→∞a n =lim n→∞b n .6、设数列{a n }满足:存在正数M ,对一切n 有: A n =|a 2-a 1|+|a 3-a 2|+…+|a n -a n-1|≤M 证明:{a n }与{A n }都收敛。
证:由A n =|a 2-a 1|+|a 3-a 2|+…+|a n -a n-1|,知A n+1-A n =|a n+1-a n |≥0,又A n ≤M , ∴{A n }递增且有上界,∴{A n }收敛.由柯西收敛准则知,对任给的ε>0,存在N ,当m>n>N 时,有|A m -A n |<ε,∴当m>n>N 时,|a m -a n |=|a m -a m-1+a m-1-a m-2…+a n+1-a n | ≤|a m -a m-1|+|a m-1-a m-2|…+|a n+1-a n |=|A m -A n |<ε, ∴{a n }满足柯西收敛准则条件. ∴{a n }收敛.7、设a>0, σ>0, a 1=12(a +σa ), a n+1=12(a n +σa n), n=1,2,…证明:数列{a n }收敛,且其极限为√σ. 证:a n+1=12(a n +σa n)=√σ2(n√σ+√σa n)≥√σ;∴{a n }有下界;又a n+1=12(a n +σa n)=a n2(1+√σa n2)≤a n2(1+√σ√σ)=a n ;∴{a n }递减;由单调有界定理可知{a n }有极限,即{a n }收敛. 设lim n→∞a n =A .对a n+1=12(a n +σa n)两边令n →∞取极限得:A=12(A +σA ),解得A=±√σ;由保号性定理知,A=√σ;即lim n→∞a n =√σ.8、设a 1>b 1>0,记a n =a n−1+b n−12,b n =2a n−1b n−1an−1+b n−1,n=2,3,…证明:数列{a n }与{b n }的极限都存在且等于√a 1b 1. 证:∵a 1>b 1>0,可设a k >b k >0,∴a k +b k >a k -b k >0; 则a k+1-b k+1=a k +b k 2−2a k b kak +b k=(a k +b k )2−4a k b k2(a k +b k )=(a k −b k )22(ak +b k )>0,∴对一切的n 有a n >b n .∴{a n }有下界;{b n }有上界. 又a n+1-a n =a n +b n2−a n =b n −a n2<0,∴{a n }递减; b n+1-b n =2a n b nan +b n−b n =b n (a n −b n )a n +b n>0,∴{b n }递增.由单调有界定理可知数列{a n }与{b n }的极限都存在. 设lim n→∞a n =a ,lim n→∞b n =b. 由a n+1=a n +b n2,两边令n →∞取极限得:a=a+b 2,化简得a=b.由a n+1b n+1=a n +b n2·2a n b nan +b n=a n b n =a n-1b n-1=…= a 1b 1,两边令n →∞取极限得:ab=a 1b 1.∴a=b=√a 1b 1;即lim n→∞a n =lim n→∞b n =√a 1b 1.9、按柯西收敛准则叙述数列{a n}发散的条件,并用它证明下列数列{a n}是发散的:(1)a n=(-1)n n;(2)a n=sin nπ2;(3)a n=1+12+…+1n.解:数列{a n}发散的充要条件:存在ε0>0,对∀的自然数N,有n0>m0>N,使|a n0-a m|≥ε0.证:(1)取ε0=12>0,对任意自然数N,取n0=N+2,m0=N+1,则|a n0-a m|≥|a n|-|a m|=(N+2)-(N+1)=1>ε0,∴{a n}发散.(2)取ε0=12>0,对任意自然数N,取n0=4N+1,m0=4N,则|a n0-a m|=|sin(4N+1)π2−sin4Nπ2|=1>ε0,∴{a n}发散.(3)取ε0=12>0,对任意自然数N,取m0>N,n0=2m0,则|a n0-a m|=(1m0+1+1m0+2+⋯+12m0)>m0·12m0=12=ε0,∴{a n}发散.10、设limn→∞a n=a,limn→∞b n=b. 设S n=max{a n,b n},T n=min{a n,b n},n=1,2,…证明:(1)limn→∞S n=max{a,b};(2)limn→∞T n=min{a,b}.证:若a=b,则max{a,b}=min{a,b}=a,记数列{C n}:a1,b1,a2,b2…a n,b n. 则limn→∞C n=a.∵{S n},{T n}都是{C n}的一个子列,∴limn→∞S n=limn→∞T n=a.若a≠b,不妨设a>b,则由保号性定理知,存在自然数N,当n>N时,有a n>b n,于是limn→∞S n=limn→∞a n=a=max{a,b};limn→∞T n=limn→∞b n=b=min{a,b}.。