9.6E: Exercises for Section 9.6
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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
- limn→∞an+1an=0. Converges by the Ratio Test.
2) ∞∑n=110nn!
3) ∞∑n=1n22n
- Answer
- limn→∞an+1an=limn→∞12(n+1n)2=12<1. Converges by the Ratio Test.
4) ∞∑n=1n102n
5) ∞∑n=1(n!)3(3n)!
- Answer
- limn→∞an+1an=127<1. Converges by the Ratio Test.
6) ∞∑n=123n(n!)3(3n)!
7) ∞∑n=1(2n)!n2n
- Answer
- limn→∞an+1an=4e2<1. Converges by the Ratio Test.
8) ∞∑n=1(2n)!(2n)n
9) ∞∑n=1n!(n/e)n
- Answer
- limn→∞an+1an=1. Ratio test is inconclusive.
10) ∞∑n=1(2n)!(n/e)2n
11) ∞∑n=1(2nn!)2(2n)2n
- Answer
- limn→∞anan+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=(k−12k+3)k
13) ak=(2k2−1k2+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
- limn→∞a1/nn=limn→∞1e+1n=1e<1. Converges by the Root Test.
20) ak=1(1+lnk)k
21) an=(ln(1+lnn))n(lnn)n
- Answer
- limn→∞a1/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!1⋅3⋅5⋯(2k−1)
23) ak=2⋅4⋅6⋯2k(2k)!
- Answer
- limk→∞ak+1ak=limk→∞12k+1=0. Converges by the Ratio Test.
24) ak=1⋅4⋅7⋯(3k−2)3kk!
25) an=(1−1n)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
- limk→∞a1/kk=ln(3)>1. Diverges by the Root Test.
28) an=(n1/n−1)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|=limn→∞32n+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 limn→∞xn=√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(n−1)n(n+1)n
- Answer
- limn→∞an=1/e2≠0. Series diverges by the Divergence Test.
36) an=(1+1n2)n (Hint: (1+1n2)n2≈e.)
37) ak=1/2sin2k
- Answer
- Terms do not tend to zero: ak≥1/2, since sin2x≤1.
38) ak=2−sin(1/k)
39) an=1/(n+2n) where (nk)=n!k!(n−k)!
- 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=2k1⋅2⋯k(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)lnk≈e−lnk.)
43) ak=(kk+lnk)2k(Hint: ak=(1+lnkk)−(k/lnk)lnk2.)
- Answer
- ak≈e−lnk2=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, n∑k=1ak(bk+1−bk)=[an+1bn+1−a1b1]−n∑k=1bk+1(ak+1−ak), to find the sum of the given series.
44) ∞∑k=1k2k (Hint: Take ak=k and bk=21−k.)
45) ∞∑k=1kck, where c>1 (Hint: Take ak=k and bk=c1−k/(c−1).)
- Answer
- If bk=c1−k/(c−1) and ak=k, then bk+1−bk=−c−k and ∞∑n=1kck=a1b1+1c−1∞∑k=1c−k=c(c−1)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=1r√n converge? (Hint: sum∞n=1an=∞∑k=1(k+1)2−1∑n=k2an.)
- Answer
- 0<r<1. Note that the ratio and root tests are inconclusive. Using the hint, there are 2k terms r√n for k2≤n<(k+1)2, and for r<1 each term is at least rk. Thus, ∞∑n=1r√n=∞∑k=1(k+1)2−1∑n=k2r√n≥∞∑k=12krk, which converges by the ratio test for r<1. For r≥1 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 limn→∞a2nan<1/2, then ∑an converges, while if limn→∞a2n+1an>1/2, then ∑an diverges.
60) Let an=143658⋯2n−12n+2=1⋅3⋅5⋯(2n−1)2n(n+1)!. Explain why the ratio test cannot determine convergence of ∞∑n=1an. Use the fact that 1−1/(4k) is increasing k to estimate limn→∞a2nan.
61) Let an=11+x22+x⋯nn+x1n=(n−1)!(1+x)(2+x)⋯(n+x). Show that a2n/an≤e−x/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=12⋅n+1n+1+xn+2n+2+x⋯2n2n+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))−n≈e−x/2. Thus for x>0,a2nan≤12e−x/2. The series converges for x>0.
62) Let an=nlnn(lnn)n. Show that a2nan→0 as n→∞.