Calculus- Early Transcendentals, 2021a

9.4. Alternating Series 353
Exercise 9.3.10 Find an N so that
Exercise 9.3.11 Find an N so that
Exercise 9.3.12 Find an N so that
9.4 Alternating Series
∞
∑
n=0
N N
1
e n is between 1
∑
n=0
e n and ∑
n=0
1
e n + 10−4 .
∞
N N
lnn
∑
n=1
n 2 is between lnn
∑
n=1
n 2 and lnn
∑
n=1
n 2 + 0.005.
∞
∑
n=2
N N
1
n(lnn) 2 is between 1
∑
n=2
n(lnn) 2 and ∑
n=2
1
n(lnn) 2 + 0.005.
Next we consider series with both positive and negative terms, but in a regular pattern: they alternate, as
in the alternating harmonic series:
∞
(−1)
∑
n−1
= 1
n=1
n 1 + −1
2 + 1 3 + −1
4 + ···= 1 1 − 1 2 + 1 3 − 1 4 + ···.
In this example the magnitude of the terms decrease, that is, |a n | forms a decreasing sequence, although
this is not required in an alternating series. Recall that for a series with positive terms, if the limit of the
terms is not zero, the series cannot converge; but even if the limit of the terms is zero, the series still may
not converge. It turns out that for alternating series, the series converges exactly when the limit of the
terms is zero. In Figure 9.4, we illustrate what happens to the partial sums of the alternating harmonic
series. Because the sizes of the terms a n are decreasing, the odd partial sums s 1 , s 3 , s 5 , and so on, form
a decreasing sequence that is bounded below by s 2 , so this sequence must converge. Likewise, the even
partial sums s 2 , s 4 , s 6 , and so on, form an increasing sequence that is bounded above by s 1 , so this sequence
also converges. Since all the even numbered partial sums are less than all the odd numbered ones, and
since the “jumps” (that is, the a i terms) are getting smaller and smaller, the two sequences must converge
to the same value, meaning the entire sequence of partial sums s 1 ,s 2 ,s 3 ,... converges as well.
1
4
s 2 = 1 2
s 6
.
.
.
a 2 = − 1 2
a 4
a 6
s 4 s 5 s 3
a 5
..
..
a 3
Figure 9.4: The alternating harmonic series.
1 = s 1 = a 1
The same argument works for any alternating sequence with terms that decrease in absolute value. The
alternating series test is worth calling a theorem.