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Mathematics LibreTexts

9.6E: Exercises for Section 9.6

  • Gilbert Strang & Edwin “Jed” Herman
  • OpenStax

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In exercises 1 - 11, use the ratio test to determine whether each series n=1an converges or diverges. State if the ratio test is inconclusive.

1) n=11n!

Answer
limnan+1an=0. Converges by the Ratio Test.

2) n=110nn!

3) n=1n22n

Answer
limnan+1an=limn12(n+1n)2=12<1. Converges by the Ratio Test.

4) n=1n102n

5) n=1(n!)3(3n)!

Answer
limnan+1an=127<1. Converges by the Ratio Test.

6) n=123n(n!)3(3n)!

7) n=1(2n)!n2n

Answer
limnan+1an=4e2<1. Converges by the Ratio Test.

8) n=1(2n)!(2n)n

9) n=1n!(n/e)n

Answer
limnan+1an=1. Ratio test is inconclusive.

10) n=1(2n)!(n/e)2n

11) n=1(2nn!)2(2n)2n

Answer
limnanan+1=1e2<1. Converges by the Ratio Test.

In exercises 12 - 21, use the root test to determine whether n=1an converges, where an is as follows.

12) ak=(k12k+3)k

13) ak=(2k21k2+3)k

Answer
limk(ak)1/k=2>1. Diverges by the Root Test.

14) an=(lnn)2nnn

15) an=n/2n

Answer
limn(an)1/n=1/2<1. Converges by the Root Test.

16) an=n/en

17) ak=keek

Answer
limk(ak)1/k=1/e<1. Converges by the Root Test.

18) ak=πkkπ

19) an=(1e+1n)n

Answer
limna1/nn=limn1e+1n=1e<1. Converges by the Root Test.

20) ak=1(1+lnk)k

21) an=(ln(1+lnn))n(lnn)n

Answer
limna1/nn=limn(ln(1+lnn))(lnn)=0 by L’Hôpital’s rule. Converges by the Root Test.

In exercises 22 - 28, use either the ratio test or the root test as appropriate to determine whether the series k=1ak with given terms ak converges, or state if the test is inconclusive.

22) ak=k!135(2k1)

23) ak=2462k(2k)!

Answer
limkak+1ak=limk12k+1=0. Converges by the Ratio Test.

24) ak=147(3k2)3kk!

25) an=(11n)n2

Answer
limn(an)1/n=1/e. Converges by the Root Test.

26) ak=(1k+1+1k+2++12k)k(Hint: Compare a1/kk to 2kkdtt.)

27) ak=(1k+1+1k+2++13k)k

Answer
limka1/kk=ln(3)>1. Diverges by the Root Test.

28) an=(n1/n1)n

In exercises 29 - 30, use the ratio test to determine whether n=1an converges, or state if the ratio test is inconclusive.

29) n=13n22n3

Answer
limn|an+1an|=limn32n+123n2+3n+1=0. Converges by the Ratio Test.

30) n=12n2nnn!

In exercise 31, use the root and limit comparison tests to determine whether n=1an converges.

31) n=11xnn where xn+1=12xn+1xn,x1=1 (Hint: Find limit of xn.)

Answer
Converges by the Root Test and Limit Comparison Test since limnxn=2.

In exercises 32 - 43, use an appropriate test to determine whether the series converges.

32) n=1n+1n3+n2+n+1

33) n=1(1)n+1(n+1)n3+3n2+3n+1

Answer
Converges absolutely by limit comparison with p−series, p=2.

34) n=1(n+1)2n3+(1.1)n

35) n=1(n1)n(n+1)n

Answer
limnan=1/e20. Series diverges by the Divergence Test.

36) an=(1+1n2)n (Hint: (1+1n2)n2e.)

37) ak=1/2sin2k

Answer
Terms do not tend to zero: ak1/2, since sin2x1.

38) ak=2sin(1/k)

39) an=1/(n+2n) where (nk)=n!k!(nk)!

Answer
an=2(n+1)(n+2), which converges by comparison with p−series for p=2.

40) ak=1/(2kk)

41) ak=2k/(3kk)

Answer
ak=2k12k(2k+1)(2k+2)3k(2/3)k converges by comparison with geometric series.

42) ak=(kk+lnk)k(Hint: ak=(1+lnkk)(k/lnk)lnkelnk.)

43) ak=(kk+lnk)2k(Hint: ak=(1+lnkk)(k/lnk)lnk2.)

Answer
akelnk2=1/k2. Series converges by limit comparison with p−series, p=2.

The series in exercises 44 - 47 converge by the ratio test. Use summation by parts, nk=1ak(bk+1bk)=[an+1bn+1a1b1]nk=1bk+1(ak+1ak), to find the sum of the given series.

44) k=1k2k (Hint: Take ak=k and bk=21k.)

45) k=1kck, where c>1 (Hint: Take ak=k and bk=c1k/(c1).)

Answer
If bk=c1k/(c1) and ak=k, then bk+1bk=ck and n=1kck=a1b1+1c1k=1ck=c(c1)2.

46) n=1n22n

47) n=1(n+1)22n

Answer
6+4+1=11

The kth term of each of the following series has a factor xk. Find the range of x for which the ratio test implies that the series converges.

48) k=1xkk2

49) k=1x2kk2

Answer
|x|1

50) k=1x2k3k

51) k=1xkk!

Answer
|x|<

52) Does there exist a number p such that n=12nnp converges?

53) Let 0<r<1. For which real numbers p does n=1nprn converge?

Answer
All real numbers p by the Ratio Test.

54) Suppose that limn|an+1an|=p. For which values of p must n=12nan converge?

55) Suppose that limn|an+1an|=p. For which values of r>0 is n=1rnan guaranteed to converge?

Answer
r<1/p

56) Suppose that |an+1an|(n+1)p for all n=1,2, where p is a fixed real number. For which values of p is n=1n!an guaranteed to converge?

57) For which values of r>0, if any, does n=1rn converge? (Hint: sumn=1an=k=1(k+1)21n=k2an.)

Answer
0<r<1. Note that the ratio and root tests are inconclusive. Using the hint, there are 2k terms rn for k2n<(k+1)2, and for r<1 each term is at least rk. Thus, n=1rn=k=1(k+1)21n=k2rnk=12krk, which converges by the ratio test for r<1. For r1 the series diverges by the divergence test.

58) Suppose that |an+2an|r<1 for all n. Can you conclude that n=1an converges?

59) Let an=2[n/2] where [x] is the greatest integer less than or equal to x. Determine whether n=1an converges and justify your answer.

Answer
One has a1=1,a2=a3=1/2,a2n=a2n+1=1/2n. The ratio test does not apply because an+1/an=1 if n is even. However, an+2/an=1/2, so the series converges according to the previous exercise. Of course, the series is just a duplicated geometric series.

The following advanced exercises use a generalized ratio test to determine convergence of some series that arise in particular applications when tests in this chapter, including the ratio and root test, are not powerful enough to determine their convergence. The test states that if limna2nan<1/2, then an converges, while if limna2n+1an>1/2, then an diverges.

60) Let an=1436582n12n+2=135(2n1)2n(n+1)!. Explain why the ratio test cannot determine convergence of n=1an. Use the fact that 11/(4k) is increasing k to estimate limna2nan.

61) Let an=11+x22+xnn+x1n=(n1)!(1+x)(2+x)(n+x). Show that a2n/anex/2/2. For which x>0 does the generalized ratio test imply convergence of n=1an? (Hint: Write 2a2n/an as a product of n factors each smaller than 1/(1+x/(2n)).)

Answer
a2n/an=12n+1n+1+xn+2n+2+x2n2n+x. The inverse of the kth factor is (n+k+x)/(n+k)>1+x/(2n) so the product is less than (1+x/(2n))nex/2. Thus for x>0,a2nan12ex/2. The series converges for x>0.

62) Let an=nlnn(lnn)n. Show that a2nan0 as n.


This page titled 9.6E: Exercises for Section 9.6 is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Gilbert Strang & Edwin “Jed” Herman (OpenStax) via source content that was edited to the style and standards of the LibreTexts platform.

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