Sequences and Series 13 Copyright © Cengage Learning. All rights reserved.

13.3 Geometric Sequences

3 Objectives ■ Geometric Sequences ■ Partial Sums of Geometric Sequences ■ What Is an Infinite Series? ■ Infinite Geometric Series

4 Geometric Sequences

5 An arithmetic sequence is generated when we repeatedly add a number d to an initial term a. A geometric sequence is generated when we start with a number a and repeatedly multiply by a fixed nonzero constant r.

6 Geometric Sequences The number r is called the common ratio because the ratio of any two consecutive terms of the sequence is r.

7 Example 1 – Geometric Sequences (a) If a = 3 and r = 2, then we have the geometric sequence 3, 3  2, 3  2 2, 3  2 3, 3  2 4,... or 3, 6, 12, 24, 48,... Notice that the ratio of any two consecutive terms is r = 2. The nth term is a n = 3(2) n – 1.

8 Example 1 – Geometric Sequences (b) The sequence 2, –10, 50, –250, 1250,... is a geometric sequence with a = 2 and r = –5. When r is negative, the terms of the sequence alternate in sign. The nth term is a n = 2(–5) n – 1. cont’d

9 Example 1 – Geometric Sequences (c) The sequence is a geometric sequence with a = 1 and r = The nth term is. cont’d

10 Example 1 – Geometric Sequences (d) The graph of the geometric sequence defined by is shown in Figure 1. Notice that the points in the graph lie on the graph of the exponential function y =. If 0 1, then the terms increase. Figure 1 cont’d

11 Geometric Sequences We can find the nth term of a geometric sequence if we know any two terms, as the next example shows.

12 Example 2 – Finding Terms of a Geometric Sequence Find the common ratio, the first term, the nth term, and the eighth term of the geometric sequence 5, 15, 45, 135,... Solution: To find a formula for the nth term of this sequence, we need to find the first term a and the common ratio r. Clearly, a = 5. To find r, we find the ratio of any two consecutive terms. For instance, r = = 3.

13 Example 2 – Solution Thus a n = 5(3) n – 1 The eighth term is a 8 = 5(3) 8 – 1 = 5(3) 7 = 10,935. a n = ar n – 1 cont’d

14 Partial Sums of Geometric Sequences

15 Partial Sums of Geometric Sequences For the geometric sequence a, ar, ar 2, ar 3, ar 4,..., ar n – 1,..., the nth partial sum is S n = = a + ar + ar 2 + ar 3 + ar 4 + · · · + ar n – 1 To find a formula for S n, we multiply S n by r and subtract from S n. S n = a + ar + ar 2 + ar 3 + ar 4 + · · · + ar n – 1 rS n = ar + ar 2 + ar 3 + ar 4 + · · · + ar n – 1 + ar n S n – rS n = a – ar n

16 Partial Sums of Geometric Sequences So S n (1 – r) = a(1 – r n ) S n = (r ≠ 1) We summarize this result.

17 Example 4 – Finding a Partial Sum of a Geometric Sequence Find the following partial sum of a geometric sequence: Solution: For this sequence a = 1 and r = 4, so a n = 4 n – 1. Since 4 6 = 4096, we use the formula for S n with n = 7, and we have Thus this partial sum is 5461.

18 What Is an Infinite Series?

19 What Is an Infinite Series? An expression of the form = a 1 + a 2 + a 3 + a is called an infinite series. The dots mean that we are to continue the addition indefinitely. As n gets larger and larger, we are adding more and more of the terms of this series. Intuitively, as n gets larger, S n gets closer to the sum of the series.

20 What Is an Infinite Series? Now notice that as n gets large, 1/2 n gets closer and closer to 0. Thus S n gets close to 1 – 0 = 1. We can write S n  1 as n  In general, if S n gets close to a finite number S as n gets large, we say that the infinite series converges (or is convergent). The number S is called the sum of the infinite series. If an infinite series does not converge, we say that the series diverges (or is divergent).

21 Infinite Geometric Series

22 Infinite Geometric Series An infinite geometric series is a series of the form a + ar + ar 2 + ar 3 + ar ar n – We can apply the reasoning used earlier to find the sum of an infinite geometric series. The nth partial sum of such a series is given by the formula (r ≠ 1) It can be shown that if | r | < 1, then r n gets close to 0 as n gets large (you can easily convince yourself of this using a calculator).

23 Infinite Geometric Series It follows that S n gets close to a / (1 – r ) as n gets large, or S n  as n  Thus the sum of this infinite geometric series is a / (1 – r ).

24 Example 6 – Infinite Series Determine whether the infinite geometric series is convergent or divergent. If it is convergent, find its sum. (a) (b) Solution: (a) This is an infinite geometric series with a = 2 and r =. Since | r | = | | < 1, the series converges. By the formula for the sum of an infinite geometric series we have

25 Example 6 – Solution (b) This is an infinite geometric series with a = 1 and r =. Since | r | = | | > 1, the series diverges. cont’d