James Stewart-Calculus_ Early Transcendentals-Cengage Learning (2015)

Section 11.1 Sequences 695
powers of 5, so a n has denominator 5 n . The signs of the terms are alternately positive
and negative, so we need to multiply by a power of 21. In Example 1(b) the factor
s21d n meant we started with a negative term. Here we want to start with a positive term
and so we use s21d n21 or s21d n11 . Therefore
a n − s21d n21 n 1 2
5 n n
Example 3 Here are some sequences that don’t have a simple defining equation.
(a) The sequence hp n j, where p n is the population of the world as of January 1 in the
year n.
(b) If we let a n be the digit in the nth decimal place of the number e, then ha n j is a
well-defined sequence whose first few terms are
h7, 1, 8, 2, 8, 1, 8, 2, 8, 4, 5, . . .j
(c) The Fibonacci sequence h f n j is defined recursively by the conditions
f 1 − 1 f 2 − 1 f n − f n21 1 f n22 n > 3
Each term is the sum of the two preceding terms. The first few terms are
h1, 1, 2, 3, 5, 8, 13, 21, . . .j
This sequence arose when the 13th-century Italian mathematician known as Fibonacci
solved a problem concerning the breeding of rabbits (see Exercise 83).
n
0 1
1
2
FIGURE 1
a¡ a a£ a¢
A sequence such as the one in Example 1(a), a n − nysn 1 1d, can be pictured either
by plotting its terms on a number line, as in Figure 1, or by plotting its graph, as in Figure
2. Note that, since a sequence is a function whose domain is the set of positive integers,
its graph consists of isolated points with coordinates
s1, a 1 d s2, a 2 d s3, a 3 d . . . sn, a n d . . .
a n
1
1
7
a= 8
From Figure 1 or Figure 2 it appears that the terms of the sequence a n − nysn 1 1d
are approaching 1 as n becomes large. In fact, the difference
1 2 n
n 1 1 − 1
n 1 1
0
2 3 4 5 6 7
n
FIGURE 2
can be made as small as we like by taking n sufficiently large. We indicate this by writing
In general, the notation
lim
n l `
n
n 1 1 − 1
lim
n l ` an − L
means that the terms of the sequence ha n j approach L as n becomes large. Notice that the
following definition of the limit of a sequence is very similar to the definition of a limit
of a function at infinity given in Section 2.6.
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.