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Introduction to Sequences - At A Glance:

We already know that an arithmetic sequence is one where the difference between successive terms is constant. The distance each term is the same. A geometric sequence is a lot like an arithmetic sequence, but it is completely different at the same time. We can think of it as the doppelgänger of the arithmetic sequence, if we like.

In a geometric sequence, the ratio between successive terms is constant. Geometric sequences grow or shrink at the same ratio from one term to the next. If we divide any two consecutive terms, we will always get the same ratio.

So if we divide the first terms in a geometric sequence of burritos and get a pinto bean, then dividing burrito-terms two and three of the geometric series will give us a pinto bean of the same size. Let's hope we all like pinto beans.

Sample Problem

Is the sequence

2,4,8,16,...

geometric?

Answer.

Look at the ratios of successive terms:

Since these ratios are all the same, the sequence is geometric.

Sample Problem

Is the sequence

5,10,15,20,...

geometric?

Answer.

Look at the ratios of successive terms:

We don't have to go any farther. The ratios are not all the same, so this sequence is not geometric.

Mom always said we should try to keep a positive attitude, but it's okay for the ratio of a geometric series to be negative. The ratio can be anything nonzero.

When the ratio is negative, successive terms change signs, kind of like the flip-flopping snap of a flag waving in the breeze, always pointing one direction and then the other.

Sample Problem

Is the sequence

3,-6,12,-24,...

geometric?

Answer. Let's look at the ratios between successive terms:

Since the ratios are all -2, this IS a geometric sequence.

We mentioned before that an arithmetic sequence is usually defined in terms of subtraction, we can also think of it in terms of addition. To get from one term to the next, you always add the same thing.

Similarly, while a geometric sequence is defined in terms of division, we can also think of it in terms of multiplication. We said that in a geometric sequence, to get from one term to the previous term, you always divide by the same thing. This is the same as saying that to get from one term to the next, you always multiply by the same thing. This "thing" isn't a swamp monster that has come from the deep to scare little children on Halloween. It is called the common ratio of the sequence and is usually denoted r.

As with the arithmetic sequences, we usually start the geometric sequences off at a1. If we know a1 and r, we know everything there is to know about a geometric sequence. We are the masters of the geometric sequence, and maybe the swamp monster too.

Sample Problem

A geometric sequence has a1 = 4 and r = 2. Find the first four terms of the sequence.

Answer.

We're given a1. To get the next term, we multiply the term we're on by r = 2.

a2 = a1(2) = 8

a3 = a2(2) = 16

a4 = a3(2) = 32

Like eating a piece of cake, we figured out the general formula on our own. To get from a1 to an we have to multiply by r a total of (n – 1) times.

(If it's not quite clear why it's (n – 1) times, go read about steps in arithmetic sequences).

Multiplying by r a total of (n – 1) times is the same thing as multiplying by rn – 1, by the definition of what exponents are.

If we start at a1 and multiply by rn – 1, we end up at

an = a1rn – 1.

Because mathematicians need to save their writing hand for scribing infinite sequence strings, it's common to abbreviate a1 by a. so you'll probably see this formula written

an = arn – 1.

In everything we do with geometric sequences from here on, the letter a refers to the first term of the sequence.

Now that we have this nice formula, if we know a and r for a geometric sequence, we can easily find any term we like.

Just like an arithmetic sequence, if we're given two consecutive terms of a geometric sequence we can work backward to find r and then a. We know that r is the ratio of successive terms:

Once we know r we can use the formula

an = arn – 1

to find a.

Just when we thought we had it all figured out, we have a new problem. How do we find r and a if we have two non-consecutive terms am and an? This is no simple arithmetic sequence. Some people might throw their hands up in the air and go eat lunch. We are going to flex our brain muscles instead of our stomachs.

This is no impossible labyrinth to find our way out of. We only need to perform one extra operation to solve this problem, but it requires knowing how many times we multiply by r to get from am to an (assume m < n).

Let's look at our equation for a geometric sequence. The nth term is

an = arn – 1

while the mth term is

am = arm – 1

Let's divide these equations:

We can cancel the <em>a</em> out, and we will multiply by , which is just 1 in disguise. Sneaky devil. This gives us

So we get 

So, once we know how many steps we need to take, we have (nm). We divide an by am, and then we take the (nm)th root. That gives us r. Then we proceed as before to find a. Does your brain feel bigger yet?

1) Find .

2) Take the (nm)th root to get r.

3) Use the geometric sequence formula to find a.

Remember that we should always check our answers by using the values of a and r you found in the formula. If we found the right values, we should be able to use the formula to get the terms am and an that we started with.

As with an arithmetic sequence, a geometric sequence is completely determined by two pieces of information. If you know a and r, you know everything there is to know about the sequence. If you know any two terms am and an, you can find r and a, which means you are the master of the geometric sequence.

Example 1

A geometric sequence has a1 = 1 and r = 2. Find the first four terms of the sequence.


Example 2

A geometric sequence has a1 = 5 and r = 2. Find

(a) a2

(b) a3

(c) a4

(d) an


Example 3

A geometric sequence has a = 2 and r = 10. Find a5.


Example 4

A geometric sequence has a9 = 40 and a10 = 80. Find a and r.


Example 5

Find a and r for the geometric sequence with a8 = -0.78125 and a9 = 0.390625.


Example 6

Find a and r for the geometric sequence with a5 = 81 and a9 = 410.0625.


Example 7

Find a and r for the geometric sequence with a5 = 6.561 and a8 = 4.782969.


Exercise 1

Determine whether the sequence is geometric.

1,

Exercise 2

Determine whether the sequence is geometric.

3,6,9,12,...

Exercise 3

Determine whether the sequence is geometric.

10, 15, 22.5, 33.75, ...

Exercise 4

Determine whether the sequence is geometric.

3, 12, 48, 192, ...

Exercise 5

Determine whether the sequence is geometric.

.9, .8, .7, .6, ...

Exercise 6

Determine if the sequence is geometric or not.

1,-1,1,-1,...

Exercise 7

Determine if the sequence is geometric or not.

-2,14,-98, 686, ...

Exercise 8

Determine if the sequence is geometric or not.

3, -15, -75, 375, ...

Exercise 9

Find the first four terms of the geometric sequence with

a1 = 7 and r = 3

Exercise 10

Find the first four terms of the geometric sequence with

a1 = 100 and

Exercise 11

Find the first four terms of the geometric sequence with

a1 = 2 and r = 0.5

Exercise 12

Find the first four terms of the geometric sequence with

a1 = 8 and r = -4

Exercise 13

Find the first four terms of the geometric sequence with

a1 = 3 and r = 1

Exercise 14

A geometric sequence has a1 = 4 and r = 5. Find

a) a2

b) a3

c) a4

d) an

Exercise 15

A geometric sequence has a1 = 3 and . Find

a) a2

b) a3

c) a4

d) an

Exercise 16

A general geometric sequence has first term a1 and common ratio r. Find

a) a2

b) a3

c) a4

d) an, the general formula for the nth term.

Exercise 17

A geometric sequence has a = 8 and r = 0.2. Find a5.

Exercise 18

A geometric sequence has a = 2 and r = -5. Find a7.

Exercise 19

A geometric sequence has a = 0.125 and r = 2. Find a20.

Exercise 20

Find a and r for the geometric sequence with the given terms.

a7 = 640 and a8 = -1280

Exercise 21

Find a and r for the geometric sequence with the given terms.

a4 = 1.2 and a5 = 0.24

Exercise 22

Find a and r for the geometric sequence with the given terms.

a4 = -3.125 and a5 = 0.78125

Exercise 23

Find a and r for the geometric sequence with the given terms.

a11 = 1 and a13 = 0.0625

Exercise 24

Find a and r for the geometric sequence with the given terms.

a4 = 51.2 and a7 = 26.2144

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