1. Copyright © 2014, 2010, 2007 Pearson Education, Inc.
Chapter 11
Sequences, Induction, and Probability
11.3 Geometric Sequences and Series
Copyright © 2014, 2010, 2007 Pearson Education, Inc. 1

2. Objectives:
Find the common ratio of a geometric sequence.
Write terms of a geometric sequence.
Use the formula for the general term of a geometric sequence.
Use the formula for the sum of the first n terms of a geometric sequence.
Find the value of an annuity.
Use the formula for the sum of an infinite geometric series.

3. Definition of a Geometric Sequence
A geometric sequence is a sequence in which each term after the first is obtained by multiplying the preceding term by a fixed nonzero constant. The amount by which we multiply each time is called the common ratio of the sequence.

4. Example: Writing the Terms of a Geometric Sequence
Write the first six terms of the geometric sequence with first term 12 and common ratio

5. General Term of a Geometric Sequence
The nth term (the general term) of a geometric sequence with first term a1 and common ratio r is

6. Example: Using the Formula for the General Term of a Geometric Sequence
Find the seventh term of the geometric sequence whose first term is 5 and whose common ratio is –3.

7. Example: Writing the General Term for a Geometric Sequence
Write the general term for the geometric sequence
3, 6, 12, 24, 48, ... Then use the formula to find the eighth term.

8. The Sum of the First n Terms of a Geometric Sequence
The sum, Sn, of the first n terms of a geometric sequence is given by
in which a1 is the first term and r is the common ratio
(r 1).

9. Example: Finding the Sum of the First n Terms of a Geometric Sequence
Find the sum of the first nine terms of the geometric sequence: 2, –6, 18, –54, ...

10. The Sum of an Infinite Geometric Series

11. Example: Finding the Sum of an Infinite Geometric Series
Find the sum of the infinite geometric series: