Geometric Sequence: Formula & Examples

Instructor: David Liano
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.

President Obama Fills Out The NCAA Bracket
NCAA bracket

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, as is shown next:

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) as is shown next:

-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.

Rule for a Geometric Sequence

The nth term of a geometric sequence is identified as a(n). For instance, a(1) is the first term of the sequence, and a(7) is the seventh term of the sequence. To get from one term of a sequence to the next, we need to multiple the preceding term by the common ratio r. The rule for finding the nth term of a sequence is shown in Figure 1.

Figure 1

Notice that the first term a(1) is multiplied by r to the power of (1 - 1) or zero. Any number to the power of zero is 1, so we are just multiplying the first term by 1. As we calculate each next term, we just keep multiplying by r. The seventh term would be a(1) multiplied by r six times or r^6.

To unlock this lesson you must be a Member.
Create your account

Register for a free trial

Are you a student or a teacher?
I am a teacher
What is your educational goal?

Unlock Your Education

See for yourself why 10 million people use

Become a member and start learning now.
Become a Member  Back

Earning College Credit

Did you know… We have over 49 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Click "next lesson" whenever you finish a lesson and quiz. Got It
You now have full access to our lessons and courses. Watch the lesson now or keep exploring. Got It
You're 25% of the way through this course! Keep going at this rate,and you'll be done before you know it.