Geometric sequence

Before we show you what a geometric sequence is, let us first talk about what a sequence is. A sequence is a set of numbers that follow a pattern. We call each number in the sequence a term.

For examples, the following are sequences:

2, 4, 8, 16, 32, 64, ...

243, 81, 27, 9, 3, 1, ...

A geometric sequence is a sequence where each term is found by multiplying or dividing the same value from one term to the next. We call this value "common ratio"

Looking at 2, 4, 8, 16, 32, 64, ......., carefully helps us to make the following observation:

Geometric sequence

As you can see, each term is found by multiplying 2, a common ratio to the previous term.

Notice that we have to add 2 to the first term to get the second term, but we have to add 4 to the second term to get 8. This shows indeed that this sequence is not created by adding or subtracting a common term.

Looking at 243, 81, 27, 9, 3, 1, ... carefully helps us to make the following observation:

Geometric sequence

This time, to find each term, we divide by 3, a common ratio, from the previous term.

How to model a geometric sequence

Many geometric sequences can me modeled with an exponential function.

An exponential function is a function of the form an where a ≠ 1 and n is a variable.

Here is the trick or recipe per se!

2, 4, 8, 16, 32, 64, .......

Let n represent any term number in the sequence.
Observe that the terms of the sequence can be written as 21, 22, 23, ...

We can therefore model the sequence with the following formula: 2n

Check:

When n = 1, which represents the first term, we get 21 = 2

When n = 2, which represents the second term, we get 22 = 2 × 2 = 4

Let us try to model 243, 81, 27, 9, 3, 1, ...............

Let n represent any term number in the sequence.
Observe that the terms of the sequence can be written as 35, 34, 33, ...

We just have to model the sequence: 5, 4, 3, .....

The process will be briefly explained here. For detailed explanation, see arithmetic sequence.

The number we subtract to each term is 1

The number that comes right before 5 in the sequence is 6.

We can therefore model the sequence with the following formula:

-1× n + 6


We can therefore model 243, 81, 27, 9, 3, 1, ............... with the exponential function below:

3-n + 6

Check:

When n = 1, which represents the first term, we get 3-1 + 6= 35 = 243

When n = 2, which represents the second term, we get 3-2 + 6= 34 = 81

Test your knowledge with the quiz below:



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