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3.05 Geometric sequences with technology

Worksheet
Geometric progressions
1

State the first four terms in the sequences defined by the following recursive rules:

a

T_{n+1} = 4 T_n,\text{ } T_1 = 0.5

b

T_{n + 1} = 0.2 T_n,\text{ } T_1 = 25

c

T_n = 2 T_{n - 1},\text{ } T_5 = 32

d

T_n = \dfrac{1}{4} T_{n - 1},\text{ } T_5 = \dfrac{1}{4}

2

For each of the following geometric sequences, evaluate:

i

\dfrac{T_2}{T_1}

ii

\dfrac{T_3}{T_2}

iii

\dfrac{T_4}{T_3}

iv

T_5

a

- 9 , - 10.8 , - 12.96 , - 15.552 , \ldots

b

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

3

The first term of a sequence is 3.9 and the common ratio is 2.

a

State the 5th term.

b

Calculate the sum of the first 5 terms.

4

In a geometric progression, T_7 = \dfrac{64}{81} and T_8 = \dfrac{128}{243}.

a

Find the value of r, the common ratio in the sequence.

b

List the first three terms of the geometric progression.

Recursive rule
5

List the first 5 terms of the following sequences defined by:

a

a_1 = 2 and a_{n+1} = 3 a_n

b

a_1 = 8 and a_n = \dfrac{1}{2} a_{n - 1}

6

Consider the following recursive definition which starts from T_1: T_n = 2 T_{n - 1},\ T_5 = 32

State the first 4 terms in the sequence.

7

The first term of a geometric sequence is 7, and the fourth term is 875.

a

Find the common ratio, r, of this sequence.

b

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

c

Calculate the sum of the terms between the 4th and 9th term inclusive.

8

The first term of a geometric sequence is 6, and the third term is 96.

a

Find the possible values of the common ratio, r, of this sequence.

b

Write the recursive rule for T_n in terms of T_{n - 1} that uses the positive common ratio, and the initial term T_1.

c

Write the recursive rule for S_n in terms of S_{n - 1} that uses the positive common ratio, and the initial term S_1.

d

Determine the sum of the terms between T_4 and T_8 inclusive.

9

The fourth term of a geometric sequence is 16, and the seventh term is 128.

a

Find the common ratio, r, of this sequence.

b

Find the first term of this sequence.

c

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

d

Determine the sum of the terms between the 3rd and 10th term inclusive.

10

The third term of a geometric sequence is 7500, and the seventh term is 12.

a

Find the common ratio, r, of this sequence.

b

Find the first term of this sequence.

c

Write the recursive rule for T_n in terms of T_{n - 1} that uses the positive common ratio, and the initial term T_1.

d

Write the recursive rule for S_n in terms of S_{n - 1} that uses the positive common ratio, and the initial term S_1.

e

Calculate the sum of the first 15 terms of the sequence containing a negative common ratio. Round your answer to the nearest whole number.

11

Consider the following sequence:

3000, 600, 120, 24, \ldots

a

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

b

Determine the sum of the first 10 terms. Round your answer to the nearest whole number.

12

If the first term of a sequence is 90\,000 and the common ratio is 1.11:

a

Find the 6th term. Round your answer to three decimal places.

b

Calculate the sum of the first 6 terms. Round your answer to the nearest whole number.

Explicit rule
13

Given the nth term of the following sequences defined by the given equation:

i

Write the first four terms of the sequence.

ii

Find the common ratio.

a

T_n = 3 \times 4^{n - 1}

b

T_n = - 4 \times \left( - 3 \right)^{n - 1}

14

Consider the following sequence:

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

a

Write the general rule for T_n, the nth term of the sequence.

b

Hence, state the next three terms of the sequence.

c

Find T_9, the 9th term of the sequence.

d

Calculate the sum of the first 9 terms. Round your answer to the nearest whole number.

15

Consider the following sequence: 20, 40, 80, 160, \ldots

By finding the rule for the nth term, find the value of n corresponding to the term 640.

16

If the first term of a sequence is 27 and the common ratio is \dfrac{1}{3}, find the 10th term.

17

Consider the following sequence:

- 0.3,- 1.5,- 7.5,- 37.5,...

a

Find the formula for the nth term of the sequence.

b

Hence, find the next three terms of the sequence.

18

In a geometric progression, T_4 = - 192 and T_7 = 12\,288.

a

Determine the value of r

b

Hence determine the value of a

c

Write an expression for T_n

19

In a geometric progression, T_4 = 192 and T_9 = 196\,608.

a

Find the value of r.

b

Hence determine the value of a.

c

Write an expression for T_n.

20

Insert three positive values between 18 and \dfrac{32}{9} such that the five terms form the successive terms in a geometric progression.

21

Three consecutive positive terms of a geometric progression have a product of 125. The third term is 9 times the first. Let the common ratio be r, and the middle term be b.

a

Find the the value of b.

b

Find the three consecutive terms.

c

State the value of r.

Tables and graphs
22

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

a

If the sequence starts from n = 1, plot the first four terms on a number plane.

b

Is the relationship depicted by this graph linear, exponential or neither?

c

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

d

Find the sum of the first 10 terms. Round your answer to the nearest whole number.

23

Consider the sequence plot drawn below:

a

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

b

Is the relationship depicted by this graph linear, exponential or neither?

c

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

d

Calculate the sum of the first 12 terms. Round your answer to the nearest whole number.

1
2
3
4
5
n
-8
-6
-4
-2
2
4
6
8
10
12
14
16
T_n
24

Given the nth term of the following geometric progressions:

i

Complete the table of values.

ii

Find the common ratio.

iii

Plot the points in the table that correspond to n = 1, n = 2, n = 3 and n = 4 on a cartesian plane.

iv

State whether the joined points would form a straight line, a wave shaped curve, a parabola, or an exponential curve.

a

T_n = 2 \times 3^{n - 1}.

n123410
T_n
b

T_n = 6 \times \left( - 2 \right)^{n - 1}.

n123411
T_n
25

For the geometric progressions given by the equations below:

i

Complete the table of values.

ii

Find the common ratio.

a

T_n = 25 \times \left(\dfrac{1}{5}\right)^{n - 1}

n123410
T_n
b

T_n = - 72 \times \left( - \dfrac{4}{3} \right)^{n - 1}.

n12346
T_n
26

The given table of values represents terms in a geometric sequence:

a

Find r, the common ratio.

b

Write an expression for the general nth term of the sequence, T_n.

n149
T_n-9576-589\,824
27

Complete the missing values in the following tables that show the nth term in a geometric sequence:

a
n12345
T_n5-320
b
n12345
T_n-27-64
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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

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