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3.04 Geometric sequences

Worksheet
Geometric sequences
1

Write down the next two terms for the following sequences:

a

4, 12, 36, \ldots

b

12, - 48, 192, \ldots

c

- 1, 8, - 64, \ldots

d

- 6, 9, -\dfrac{27}{2},\ldots

2

Consider the sequence -1, -7, -49, \ldots

a

Find the next term of the sequence.

b

Find the 5th term of the sequence.

c

Find the 6th term of the sequence.

3

Explain how the common ratio of a geometric sequence can be found.

4

Suppose t_{1}, t_{2}, t_{3}, t_{4}, t_{5},\ldots is a geometric sequence.

Is t_{1}, t_{3}, t_{5},\ldots also a geometric sequence? Explain your answer.

5

Consider the first four terms of the following geometric sequences:

i

Evaluate \dfrac{T_2}{T_1}.

ii

Evaluate \dfrac{T_3}{T_2}.

iii

Evaluate \dfrac{T_4}{T_3}.

iv

Hence, find T_5.

a
- 8, - 16, - 32, - 64, \ldots
b

- 4 , - 8 , - 16 , - 32 , \ldots

c

2, - 6 , 18, - 54 , \ldots

d

- 64 , - 16 , - 4 , -1, \ldots

6

State the common ratio between the terms of the following sequences:

a

9, 36, 144, 576, \ldots

b

- 6 , - 42 , - 294 , - 2058 , \ldots

c

2, - 16, 128, - 1024, \ldots

d

- 70.4 , - 17.6 , - 4.4 , - 1.1 ,\ldots

7

Write the first 5 terms of the following sequences given the first term and the common ratio:

a

First term: - 2, common ratio: 3

b

First term: 1.3, common ratio: - 4.

c

First term: 700\,000, common ratio: 1.04.

8

For each of the following, write the first four terms in the geometric progression:

a

The first term is 6 and the common ratio is 4.

b

The first term is 7 and the common ratio is - 2.

c

The first term is 700\,000 and the common ratio is 1.04.

d

The first term is - 2 and the common ratio is 3.

e

The first term is 1.3 and the common ratio is - 4.

9

Find the missing terms in the following geometric progressions:

a

- 5, \, x, \, - 80, \, 320, \, y

b

a, \, b, \, \dfrac{3}{25}, \, - \dfrac{3}{125}, \, c

10

For each of the following pairs of terms in a geometric progression:

i

Find the possible values of r.

ii

Find the value of a.

iii

Find the general rule for T_n, for r \gt 0.

a

T_3 = 18 and T_5 = 162

b

T_4 = 32 and T_6 = 128

Recursive rules
11

For each of the following recursive rules, write the rule for the nth term of the sequence:

a

T_{n + 1} = 6 T_n, \ T_1 = 5

b

T_{n + 1} = \dfrac{1}{5} T_n, \ T_1 = 213

c

T_n = - 4 T_{n - 1}, \ T_1 = 2

12

State the first 4 terms in the following recursive definition: T_n = 4 T_{n - 1}, \ T_1 = 0.5

13

For the following sequences, write the recursive rule for T_n in terms of T_{n - 1}, including the initial term T_1:

a

9, 45, 225, 1125, \ldots

b

2, 20, 200, 2000, \ldots

c

5, - 15 , 45, - 135 , \ldots

14

For each of the following sequences:

i

Write a recursive rule for T_n in terms of T_{n - 1}, including the initial term T_1.

ii

Find the general rule for T_n.

iii

Calculate the 15th term of the sequence.

a

1, 3, 9, 27, \ldots

b

200, 100, 50, 25, \ldots

Tables and graphs
15

Each of the given tables of values represents terms in a geometric sequence:

i

Find r, the common ratio between consecutive terms.

ii

Write a simplified expression for the nth term of the sequence, T_n.

iii

Find the missing term in the table.

a
n123410
T_n5403202560
b
n123412
T_n7-2163-189
c
n136911
T_n-5-45-1215-32\,805
d
n12347
T_n-2-\dfrac {16}{3}-\dfrac {128}{9}-\dfrac {1024}{27}
16

Consider the following sequence 5, - 10 , 20, - 40 , \ldots

a

Plot the first four terms on a number plane.

b

Is the sequence arithmetic or geometric?

17

Consider the sequence: 2, 6, 18, 54, \ldots

a

Plot the first four terms on a number plane.

b

Is this sequence arithmetic, geometric or neither?

c

Write a recursive rule for T_n in terms of T_{n - 1} and an initial condition for T_1.

18

Consider the first-order recurrence relationship defined by T_n = 2 T_{n - 1},\text{ } T_1 = 2.

a

Determine the next three terms of the sequence from T_2 to T_4.

b

Plot the first four terms on a number plane.

c

Is the sequence arithmetic or geometric?

19

Consider the sequence plot drawn below:

a

State the terms of the first five points of the sequence.

b

Is the sequence arithmetic or geometric? Explain your answer.

c

Write a recursive rule for T_{n+1} in terms of T_n and an initial condition for T_1.

1
2
3
4
5
n
2
4
6
8
10
12
14
16
18
20
T_n
20

For each of the following graphs, write the recursive rule for T_n in terms of T_{n - 1}, including the initial term T_1:

a
1
2
3
4
x
10
20
30
40
50
60
70
80
y
b
1
2
3
4
5
x
5
10
15
20
25
30
35
40
45
50
55
y
c
1
2
3
4
x
-45
-40
-35
-30
-25
-20
-15
-10
-5
y
21

For each of the following:

i

Create a table of values for the given points.

ii

Find r.

iii

Write a simplified expression for T_n.

iv

Find the 10th term of the sequence.

a
1
2
3
n
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
T_n
b
1
2
3
4
5
n
5
10
15
20
25
T_n
c
1
2
3
4
n
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
5
10
15
20
T_n
22

The plotted points represent terms in a geometric sequence:

a

Identify r, the common ratio.

b

Write a simplified expression for the nth term of the sequence, T_n.

c

The points are reflected about the horizontal axis to form three new points.

If these new points represent consecutive terms of a geometric sequence, write the equation for T_k, the kth term in this new sequence.

1
2
3
4
5
n
2
4
6
8
10
12
T_n
Applications
23

Suppose you save \$1 on the first day of a month, \$2 on the second day, \$4 on the third day, \$8 on the fourth day, and so on. That is, each day you save twice as much as you did the day before.

a

How much will you put aside for savings on the 6th day of the month?

b

How much will you put aside for savings on the 10th day of the month?

24

The average daily growth of a seedling is 10\% per day. A seedling measuring 6 \text{ cm} in height is planted.

a

Determine the height of the seedling at the end of Day 1.

b

Find the height of the seedling 2 days after it is planted.

c

Write a recursive rule for H_n, defining the height of the seedling n days after it is planted, and an initial condition H_0.

25

Radioactive element D loses half its mass every day. The initial mass of the element was 800 grams.

a

Complete the given table.

b

Is the type of decay linear or exponential?

\text{Day}\text{Mass of radioactive element D (g)}
0
1
2
3
4
26

Consider the table of values:

\text{Number of days passed } (x)12345
\text{Population of shrimp } (y)5251256253125
a

Is the number of shrimp increasing by the same amount each day?

b

Find the equation linking population y and time x in the form y = a^{x}.

c

Sketch the graph of the equation.

27

In a laboratory, an antibiotic is tested on a sample of 5 bacteria in a petri dish. The number of bacteria is recorded, and the bacteria are found to double each hour.

a

Complete the table of values.

\text{Number of hours passed } (x)01234
\text{Number of bacteria } (y)520
b

Find the equation linking the number of bacteria (y) and number of hours passed (x).

c

At this rate, how many bacteria will be present in the petri dish after 18 hours? Leave your answer in exponential form.

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Outcomes

ACMGM071

use recursion to generate a geometric sequence

ACMGM072

display the terms of a geometric sequence in both tabular and graphical form and demonstrate that geometric sequences can be used to model exponential growth and decay in discrete situations

ACMGM073

deduce a rule for the nth term of a particular geometric sequence from the pattern of the terms in the sequence, and use this rule to make predictions

ACMGM074

use geometric sequences to model and analyse (numerically, or graphically only) practical problems involving geometric growth and decay; for example, analysing a compound interest loan or investment, the growth of a bacterial population that doubles in size each hour, the decreasing height of the bounce of a ball at each bounce; or calculating the value of office furniture at the end of each year using the declining (reducing) balance method to depreciate

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