Geometric Sequence: Formula & Examples

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  • 0:00 The Geometric Sequence Concept
  • 1:47 Identifying Geometric…
  • 2:54 Rule for a Geometric Sequence
  • 3:43 Writing a Rule for a…
  • 5:12 Lesson Summary
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Lesson Transcript
Instructor: David Liano

David has a Master of Business Administration, a BS in Marketing, and a BA in History.

After completing this lesson, you will be able to recognize a geometric sequence. You will also be able to use the general formula for finding a term in a geometric sequence and will be able to write a custom formula for a given geometric sequence.

The Geometric Sequence Concept

In mathematics, a sequence is usually meant to be a progression of numbers with a clear starting point. What makes a sequence geometric is a common relationship that exists between any two consecutive numbers in the sequence.

Let's consider the NCAA basketball tournament. After the preliminary rounds, the tournament has a field of 64 teams. In the round of 64, all teams play, so there will be 32 teams eliminated. In other words, there are 32 teams left, or half of what we started with. After the round of 32, there are 16 teams left. Again, the number of teams has been cut in half. This pattern continues until there is one team left. Let's write this as a sequence:

64, 32, 16, 8, 4, 2, 1

Do you see the relationship between any two consecutive terms? Each term after the first term is ½ of the preceding term. Another way to look at it is that we are multiplying each term by ½ to get the next term in the sequence. Also notice that the ratio of any term and its preceding term is ½. For example 32/64 = ½ and 2/4 = ½. This is called the common ratio of the geometric series, and it is denoted by r. This ratio must hold true for any pair of consecutive terms. Otherwise, the sequence is not a geometric sequence.

This example is a finite geometric sequence; the sequence stops at 1. Some geometric sequences continue with no end, and that type of sequence is called an infinite geometric sequence.

Identifying Geometric Sequences

Let's look at other examples of geometric sequences:

6, 12, 24, 48, 96, ...

4, -6, 9, -13.5, ...

The first sequence has a common ratio of 2:

12/6 = 24/12 = 48/24 = 96/48 = 2

The second sequence is also geometric. It might be hard to see at first, but it does have a common ratio of (-3/2):

-6/4 = 9/-6 = -13.5/9 = -3/2

Let's now look at some sequences that are not geometric:

1, 4, 9, 16, 25, ...

100, 90, 80, 70, 60, ...

In each sequence, the ratio between consecutive terms is not the same. For instance, 4/1 does not equal 9/4 in the first sequence. In the second sequence, 90/100 does not equal 80/90.

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