Geometric Sequences and Sums

Sequence

A Sequence is a set of things (usually numbers) that are in order.

Sequence

Geometric Sequences

In a Geometric Sequence each term is found by multiplying the previous term by a constant.

Example:

2, 4, 8, 16, 32, 64, 128, 256, ...

This sequence has a factor of 2 between each number.

Each term (except the first term) is found by multiplying the previous term by 2.

 

In General we write a Geometric Sequence like this:

{a, ar, ar2, ar3, ... }

where:

  • a is the first term, and
  • r is the factor between the terms (called the "common ratio")

 

Example: {1,2,4,8,...}

The sequence starts at 1 and doubles each time, so

  • a=1 (the first term)
  • r=2 (the "common ratio" between terms is a doubling)

And we get:

{a, ar, ar2, ar3, ... }

= {1, 1×2, 1×22, 1×23, ... }

= {1, 2, 4, 8, ... }

 

But be careful, r should not be 0:

  • When r=0, we get the sequence {a,0,0,...} which is not geometric

The Rule

We can also calculate any term using the Rule:

xn = ar(n-1)

(We use "n-1" because ar0 is for the 1st term)

 

Example:

10, 30, 90, 270, 810, 2430, ...

This sequence has a factor of 3 between each number.

The values of a and r are:

  • a = 10 (the first term)
  • r = 3 (the "common ratio")

The Rule for any term is:

xn = 10 × 3(n-1)

So, the 4th term is:

x4 = 10×3(4-1) = 10×33 = 10×27 = 270

And the 10th term is:

x10 = 10×3(10-1) = 10×39 = 10×19683 = 196830

 

A Geometric Sequence can also have smaller and smaller values:

Example:

4, 2, 1, 0.5, 0.25, ...

This sequence has a factor of 0.5 (a half) between each number.

Its Rule is xn = 4 × (0.5)n-1

Why "Geometric" Sequence?

Because it is like increasing the dimensions in geometry:

Geometric Sequence a line is 1-dimensional and has a length of r
in 2 dimensions a square has an area of r2
in 3 dimensions a cube has volume r3
etc (yes we can have 4 and more dimensions in mathematics).

 

Geometric Sequences are sometimes called Geometric Progressions (G.P.’s)

Summing a Geometric Series

When we need to sum a Geometric Sequence, there is a handy formula.

To sum:

a + ar + ar2 + ... + ar(n-1)

Each term is ark, where k starts at 0 and goes up to n-1

Use this formula:

Sigma

a is the first term
r is the "common ratio" between terms
n is the number of terms

What is that funny symbol? It is called Sigma Notation

Sigma (called Sigma) means "sum up"

And below and above it are shown the starting and ending values:

Sigma Notation

It says "Sum up n where n goes from 1 to 4. Answer=10

The formula is easy to use ... just "plug in" the values of a, r and n

Example: Sum the first 4 terms of

10, 30, 90, 270, 810, 2430, ...

This sequence has a factor of 3 between each number.

 

The values of a, r and n are:

  • a = 10 (the first term)
  • r = 3 (the "common ratio")
  • n = 4 (we want to sum the first 4 terms)

So:

Sigma

Becomes:

Sigma

You can check it yourself:

10 + 30 + 90 + 270 = 400

And, yes, it is easier to just add them in this example, as there are only 4 terms. But imagine adding 50 terms ... then the formula is much easier.

Using the Formula

Let's see the formula in action:

Example: Grains of Rice on a Chess Board

chess board

On the page Binary Digits we give an example of grains of rice on a chess board. The question is asked:

When we place rice on a chess board:

  • 1 grain on the first square,
  • 2 grains on the second square,
  • 4 grains on the third and so on,
  • ...

... doubling the grains of rice on each square ...

... how many grains of rice in total?

So we have:

  • a = 1 (the first term)
  • r = 2 (doubles each time)
  • n = 64 (64 squares on a chess board)

So:

Sigma

Becomes:

Sigma

 

= (1-264) / (-1) = 264 - 1

= 18,446,744,073,709,551,615

Which was exactly the result we got on the Binary Digits page (thank goodness!)

And another example, this time with r less than 1:

Example: Add up the first 10 terms of the Geometric Sequence that halves each time:

{ 1/2, 1/4, 1/8, 1/16, ... }

The values of a, r and n are:

  • a = ½ (the first term)
  • r = ½ (halves each time)
  • n = 10 (10 terms to add)

So:

Sigma

Becomes:

Sigma

Very close to 1.

(Question: if we continue to increase n, what happens?)

Why Does the Formula Work?

Let's see why the formula works, because we get to use an interesting "trick" which is worth knowing.

First, call the whole sum "S":   S =   a + ar + ar2 + ... + ar(n-2)+ ar(n-1)
       
Next, multiply S by r:   S·r =   ar + ar2 + ar3 + ... + ar(n-1) + arn

Notice that S and S·r are similar?

Now subtract them!

Proof

Wow! All the terms in the middle neatly cancel out.
(That is a neat trick!)

By subtracting S·r from S we get a simple result:

S − S·r = a − arn

Let's rearrange it to find S:

Factor out S and a:   S(1r) = a(1rn)
     
Divide by (1-r):   S = a(1rn)/(1r)

Which is our formula (ta-da!):

Sigma

 

Infinite Geometric Series

So what happens when n goes to infinity?

Well ... when r is less than 1, then rn goes to zero and we get:

Sigma

NOTE: this does not work when r is 1 or more (or less than -1):

r must be between (but not including) -1 and 1

and r should not be 0 because we get the sequence {a,0,0,...} which isn't geometric

Let's bring back our previous example, and see what happens:

Example: Add up ALL the terms of the Geometric Sequence that halves each time:

{ 1/2, 1/4, 1/8, 1/16, ... }

We have:

  • a = 1/2 (the first term)
  • r = 1/2 (halves each time)

And so:

Sigma

= ½ × 1 / ½ = 1

Yes ... adding (1/2)+(1/4)+(1/8)+... equals exactly 1.

Don't believe me? Just look at this square:

By adding up (1/2)+(1/4)+(1/8)+...

... we end up with the whole thing!

  1/2 + 1/4 + ...

Recurring Decimal

On another page we asked "Does 0.999... equal 1?", well, let us see if we can calculate it:

Example: Calculate 0.999...

We can write a recurring decimal as a sum like this:

Sigma

And now we can use the formula:

Sigma

 

Yes! 0.999... does equal 1.

 

So there we have it ... Geometric Sequences (and their sums) can do all sorts of amazing and powerful things.