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### Text of Arithmetic Sequences - OpenStax CNX...Some arithmetic sequences are de ned in terms of the previous... OpenStax-CNX module: m49445 1

Arithmetic Sequences∗

OpenStax College

This work is produced by OpenStax-CNX and licensed under the

Abstract

In this section, you will:

• Find the common di�erence for an arithmetic sequence.

• Write terms of an arithmetic sequence.

• Use a recursive formula for an arithmetic sequence.

• Use an explicit formula for an arithmetic sequence.

Companies often make large purchases, such as computers and vehicles, for business use. The book-valueof these supplies decreases each year for tax purposes. This decrease in value is called depreciation. Onemethod of calculating depreciation is straight-line depreciation, in which the value of the asset decreases bythe same amount each year.

As an example, consider a woman who starts a small contracting business. She purchases a new truckfor \$25,000. After �ve years, she estimates that she will be able to sell the truck for \$8,000. The loss invalue of the truck will therefore be \$17,000, which is \$3,400 per year for �ve years. The truck will be worth\$21,600 after the �rst year; \$18,200 after two years; \$14,800 after three years; \$11,400 after four years; and\$8,000 at the end of �ve years. In this section, we will consider speci�c kinds of sequences that will allow usto calculate depreciation, such as the truck's value.

1 Finding Common Di�erences

The values of the truck in the example are said to form an arithmetic sequence because they changeby a constant amount each year. Each term increases or decreases by the same constant value called thecommon di�erence of the sequence. For this sequence, the common di�erence is �3,400.

The sequence below is another example of an arithmetic sequence. In this case, the constant di�erenceis 3. You can choose any term of the sequence, and add 3 to �nd the subsequent term.

∗Version 1.8: Mar 11, 2015 11:08 am +0000†http://creativecommons.org/licenses/by/4.0/

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A General Note: An arithmetic sequence is a sequence that has the property that thedi�erence between any two consecutive terms is a constant. This constant is called the commondi�erence. If a1 is the �rst term of an arithmetic sequence and d is the common di�erence, thesequence will be:

{an} = {a1, a1 + d, a1 + 2d, a1 + 3d, ...} (1)

Example 1Finding Common Di�erencesIs each sequence arithmetic? If so, �nd the common di�erence.

a. {1, 2, 4, 8, 16, ...}b. {−3, 1, 5, 9, 13, ...}

SolutionSubtract each term from the subsequent term to determine whether a common di�erence exists.

a. The sequence is not arithmetic because there is no common di�erence.

b. The sequence is arithmetic because there is a common di�erence. The common di�erence is4.

AnalysisThe graph of each of these sequences is shown in Figure 1. We can see from the graphs that,although both sequences show growth, a is not linear whereas b is linear. Arithmetic sequenceshave a constant rate of change so their graphs will always be points on a line.

Figure 1

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Q&A: If we are told that a sequence is arithmetic, do we have to subtract every termfrom the following term to �nd the common di�erence?

No. If we know that the sequence is arithmetic, we can choose any one term in the sequence, and

subtract it from the subsequent term to �nd the common di�erence.

Try It:

Exercise 2 (Solution on p. 21.)

Is the given sequence arithmetic? If so, �nd the common di�erence.

{18, 16, 14, 12, 10, . . . } (2)

Try It:

Exercise 3 (Solution on p. 21.)

Is the given sequence arithmetic? If so, �nd the common di�erence.

{1, 3, 6, 10, 15, . . . } (3)

2 Writing Terms of Arithmetic Sequences

Now that we can recognize an arithmetic sequence, we will �nd the terms if we are given the �rst term andthe common di�erence. The terms can be found by beginning with the �rst term and adding the commondi�erence repeatedly. In addition, any term can also be found by plugging in the values of n and d intoformula below.

an = a1 + (n− 1) d (4)

How To: Given the �rst term and the common di�erence of an arithmetic sequence,�nd the �rst several terms.

1.Add the common di�erence to the �rst term to �nd the second term.2.Add the common di�erence to the second term to �nd the third term.3.Continue until all of the desired terms are identi�ed.4.Write the terms separated by commas within brackets.

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Example 2Writing Terms of Arithmetic SequencesWrite the �rst �ve terms of the arithmetic sequence with a1 = 17 and d = −3.

SolutionAdding − 3 is the same as subtracting 3. Beginning with the �rst term, subtract 3 from each termto �nd the next term.

The �rst �ve terms are {17, 14, 11, 8, 5}AnalysisAs expected, the graph of the sequence consists of points on a line as shown in Figure 2.

Figure 2

Try It:

Exercise 5 (Solution on p. 21.)

List the �rst �ve terms of the arithmetic sequence with a1 = 1 and d = 5.

How To: Given any the �rst term and any other term in an arithmetic sequence, �nda given term.

1.Substitute the values given for a1, an, n into the formula an = a1 + (n− 1) d to solve for d.2.Find a given term by substituting the appropriate values for a1, n, and d into the formulaan =

a1 + (n− 1) d.

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Example 3Writing Terms of Arithmetic SequencesGiven a1 = 8 and a4 = 14, �nd a5.

SolutionThe sequence can be written in terms of the initial term 8 and the common di�erence d.

{8, 8 + d, 8 + 2d, 8 + 3d} (5)

We know the fourth term equals 14; we know the fourth term has the form a1 + 3d = 8 + 3d.We can �nd the common di�erence d.

an = a1 + (n− 1) d

a4 = a1 + 3d

a4 = 8 + 3d Write the fourth term of the sequence in terms of a1 and d.

14 = 8 + 3d Substitute 14 for a4.

d = 2 Solve for the common di�erence.

(6)

Find the �fth term by adding the common di�erence to the fourth term.

a5 = a4 + 2 = 16 (7)

AnalysisNotice that the common di�erence is added to the �rst term once to �nd the second term, twiceto �nd the third term, three times to �nd the fourth term, and so on. The tenth term couldbe found by adding the common di�erence to the �rst term nine times or by using the equationan = a1 + (n− 1) d.

Try It:

Exercise 7 (Solution on p. 21.)

Given a3 = 7 and a5 = 17, �nd a2.

3 Using Recursive Formulas for Arithmetic Sequences

Some arithmetic sequences are de�ned in terms of the previous term using a recursive formula. Theformula provides an algebraic rule for determining the terms of the sequence. A recursive formula allows usto �nd any term of an arithmetic sequence using a function of the preceding term. Each term is the sum ofthe previous term and the common di�erence. For example, if the common di�erence is 5, then each termis the previous term plus 5. As with any recursive formula, the �rst term must be given.

an = an−1 + d n ≥ 2 (8)

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A General Note: The recursive formula for an arithmetic sequence with common di�erence dis:

an = an−1 + d n ≥ 2 (9)

How To: Given an arithmetic sequence, write its recursive formula.

1.Subtract any term from the subsequent term to �nd the common di�erence.2.State the initial term and substitute the common di�erence into the recursive formula forarithmetic sequences.

Example 4Writing a Recursive Formula for an Arithmetic SequenceWrite a recursive formula for the arithmetic sequence.

{−18, − 7, 4, 15, 26, . . .} (10)

Solution

The �rst term is given as −18. The common di�erence can be found by subtracting the �rst termfrom the second term.

d = −7− (−18) = 11 (11)

Substitute the initial term and the common di�erence into the recursive formula for arithmeticsequences.

a1 = −18

an = an−1 + 11, for n ≥ 2(12)

AnalysisWe see that the common di�erence is the slope of the line formed when we graph the terms of thesequence, as shown in Figure 3. The growth pattern of the sequence shows the constant di�erenceof 11 units.

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Figure 3

Q&A: Do we have to subtract the �rst term from the second term to �nd the commondi�erence?

No. We can subtract any term in the sequence from the subsequent term. It is, however, most

common to subtract the �rst term from the second term because it is often the easiest method of

�nding the common di�erence.

Try It:

Exercise 9 (Solution on p. 21.)

Write a recursive formula for the arithmetic sequence.

{25, 37, 49, 61, . . .} (13)

4 Using Explicit Formulas for Arithmetic Sequences

We can think of an arithmetic sequence as a function on the domain of the natural numbers; it is a linearfunction because it has a constant rate of change. The common di�erence is the constant rate of change,or the slope of the function. We can construct the linear function if we know the slope and the verticalintercept.

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an = a1 + d (n− 1) (14)

To �nd the y-intercept of the function, we can subtract the common di�erence from the �rst term of thesequence. Consider the following sequence.

The common di�erence is −50, so the sequence represents a linear function with a slope of −50. To �ndthe y-intercept, we subtract −50 from200 : 200− (−50) = 200 + 50 = 250. You can also �nd the y-interceptby graphing the function and determining where a line that connects the points would intersect the verticalaxis. The graph is shown in Figure 4.

Figure 4

Recall the slope-intercept form of a line is y = mx + b.When dealing with sequences, we use an in placeof y and n in place of x. If we know the slope and vertical intercept of the function, we can substitute themfor m and b in the slope-intercept form of a line. Substituting − 50 for the slope and 250 for the verticalintercept, we get the following equation:

an = −50n + 250 (15)

We do not need to �nd the vertical intercept to write an explicit formula for an arithmetic sequence.Another explicit formula for this sequence is an = 200− 50 (n− 1), which simpli�es to an = −50n + 250.

A General Note: An explicit formula for the nth term of an arithmetic sequence is given by

an = a1 + d (n− 1) (16)

How To: Given the �rst several terms for an arithmetic sequence, write an explicitformula.

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1.Find the common di�erence, a2 − a1.2.Substitute the common di�erence and the �rst term into an = a1 + d (n− 1) .

Example 5Writing the nth Term Explicit Formula for an Arithmetic SequenceWrite an explicit formula for the arithmetic sequence.

{2, 12, 22, 32, 42, . . .} (17)

Solution

The common di�erence can be found by subtracting the �rst term from the second term.

d = a2 − a1

= 12− 2

= 10

(18)

The common di�erence is 10. Substitute the common di�erence and the �rst term of the sequenceinto the formula and simplify.

an = 2 + 10 (n− 1)

an = 10n− 8(19)

AnalysisThe graph of this sequence, represented in Figure 5, shows a slope of 10 and a vertical interceptof −8.

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Figure 5

Try It:

Exercise 11 (Solution on p. 21.)

Write an explicit formula for the following arithmetic sequence.

{50, 47, 44, 41, . . . } (20)

4.1 Finding the Number of Terms in a Finite Arithmetic Sequence

Explicit formulas can be used to determine the number of terms in a �nite arithmetic sequence. We need to�nd the common di�erence, and then determine how many times the common di�erence must be added tothe �rst term to obtain the �nal term of the sequence.

How To: Given the �rst three terms and the last term of a �nite arithmetic sequence,�nd the total number of terms.

1.Find the common di�erence d.2.Substitute the common di�erence and the �rst term into an = a1 + d (n−−1) .

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3.Substitute the last term for an and solve for n.

Example 6Finding the Number of Terms in a Finite Arithmetic SequenceFind the number of terms in the �nite arithmetic sequence.

{8, 1, −−6, ..., −−41} (21)

Solution

The common di�erence can be found by subtracting the �rst term from the second term.

1− 8 = −7 (22)

The common di�erence is−7. Substitute the common di�erence and the initial term of the sequenceinto the nth term formula and simplify.

an = a1 + d (n− 1)

an = 8 +−7 (n− 1)

an = 15− 7n

(23)

Substitute −41 for an and solve for n

−41 = 15− 7n

8 = n(24)

There are eight terms in the sequence.

Try It:

Exercise 13 (Solution on p. 21.)

Find the number of terms in the �nite arithmetic sequence.

{6, 11, 16, ..., 56} (25)

4.2 Solving Application Problems with Arithmetic Sequences

In many application problems, it often makes sense to use an initial term of a0 instead of a1. In theseproblems, we alter the explicit formula slightly to account for the di�erence in initial terms. We use thefollowing formula:

an = a0 + dn (26)

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Example 7Solving Application Problems with Arithmetic SequencesA �ve-year old child receives an allowance of \$1 each week. His parents promise him an annualincrease of \$2 per week.

a. Write a formula for the child's weekly allowance in a given year.b. What will the child's allowance be when he is 16 years old?

Solution

a. The situation can be modeled by an arithmetic sequence with an initial term of 1 and acommon di�erence of 2.Let A be the amount of the allowance and n be the number of years after age 5. Using thealtered explicit formula for an arithmetic sequence we get:

An = 1 + 2n (27)

b. We can �nd the number of years since age 5 by subtracting.

16− 5 = 11 (28)

We are looking for the child's allowance after 11 years. Substitute 11 into the formula to �ndthe child's allowance at age 16.

A11 = 1 + 2 (11) = 23 (29)

The child's allowance at age 16 will be \$23 per week.

Try It:

Exercise 15 (Solution on p. 21.)

A woman decides to go for a 10-minute run every day this week and plans to increasethe time of her daily run by 4 minutes each week. Write a formula for the time of her runafter n weeks. How long will her daily run be 8 weeks from today?

Media: Access this online resource for additional instruction and practice with arithmetic se-quences.

• Arithmetic Sequences1

5 Key Equations

recursive formula for nth term of an arithmetic sequence an = an−1 + dn ≥ 2

explicit formula for nth term of an arithmetic sequence an = a1 + d (n− 1)

Table 1

1http://openstaxcollege.org/l/arithmeticseq

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6 Key Concepts

• An arithmetic sequence is a sequence where the di�erence between any two consecutive terms is aconstant.

• The constant between two consecutive terms is called the common di�erence.• The common di�erence is the number added to any one term of an arithmetic sequence that generates

the subsequent term. See Example 1.• The terms of an arithmetic sequence can be found by beginning with the initial term and adding the

common di�erence repeatedly. See Example 2 and Example 3.• A recursive formula for an arithmetic sequence with common di�erence d is given by an = an−1+d, n ≥

2. See Example 4.• As with any recursive formula, the initial term of the sequence must be given.• An explicit formula for an arithmetic sequence with common di�erence d is given by an = a1+d (n− 1) .

See Example 5.• An explicit formula can be used to �nd the number of terms in a sequence. See Example 6.• In application problems, we sometimes alter the explicit formula slightly to an = a0+dn. See Example 7.

7 Section Exercises

7.1 Verbal

Exercise 16 (Solution on p. 21.)

What is an arithmetic sequence?

Exercise 17How is the common di�erence of an arithmetic sequence found?

Exercise 18 (Solution on p. 21.)

How do we determine whether a sequence is arithmetic?

Exercise 19What are the main di�erences between using a recursive formula and using an explicit formula todescribe an arithmetic sequence?

Exercise 20 (Solution on p. 21.)

Describe how linear functions and arithmetic sequences are similar. How are they di�erent?

7.2 Algebraic

For the following exercises, �nd the common di�erence for the arithmetic sequence provided.

Exercise 21{5, 11, 17, 23, 29, ...}

Exercise 22 (Solution on p. 21.)

{0, 12 , 1, 3

2 , 2, ...}For the following exercises, determine whether the sequence is arithmetic. If so �nd the common di�erence.

Exercise 23{11.4, 9.3, 7.2, 5.1, 3, ...}

Exercise 24 (Solution on p. 21.)

{4, 16, 64, 256, 1024, ...}For the following exercises, write the �rst �ve terms of the arithmetic sequence given the �rst term andcommon di�erence.

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Exercise 25a1 = −25, d = −9Exercise 26 (Solution on p. 21.)

a1 = 0, d = 23

For the following exercises, write the �rst �ve terms of the arithmetic series given two terms.

Exercise 27a1 = 17, a7 = −31

Exercise 28 (Solution on p. 21.)

a13 = −60, a33 = −160For the following exercises, �nd the speci�ed term for the arithmetic sequence given the �rst term andcommon di�erence.

Exercise 29First term is 3, common di�erence is 4, �nd the 5th term.

Exercise 30 (Solution on p. 21.)

First term is 4, common di�erence is 5, �nd the 4th term.

Exercise 31First term is 5, common di�erence is 6, �nd the 8th term.

Exercise 32 (Solution on p. 21.)

First term is 6, common di�erence is 7, �nd the 6th term.

Exercise 33First term is 7, common di�erence is 8, �nd the 7th term.

For the following exercises, �nd the �rst term given two terms from an arithmetic sequence.

Exercise 34 (Solution on p. 21.)

Find the �rst term or a1 of an arithmetic sequence if a6 = 12 and a14 = 28.

Exercise 35Find the �rst term or a1 of an arithmetic sequence if a7 = 21 and a15 = 42.

Exercise 36 (Solution on p. 21.)

Find the �rst term or a1 of an arithmetic sequence if a8 = 40 and a23 = 115.

Exercise 37Find the �rst term or a1 of an arithmetic sequence if a9 = 54 and a17 = 102.

Exercise 38 (Solution on p. 21.)

Find the �rst term or a1 of an arithmetic sequence if a11 = 11 and a21 = 16.

For the following exercises, �nd the speci�ed term given two terms from an arithmetic sequence.

Exercise 39a1 = 33 and a7 = −15. Find a4.

Exercise 40 (Solution on p. 21.)

a3 = −17.1 and a10 = −15.7. Finda21.

For the following exercises, use the recursive formula to write the �rst �ve terms of the arithmetic sequence.

Exercise 41a1 = 39; an = an−1 − 3

Exercise 42 (Solution on p. 21.)

a1 = −19; an = an−1 − 1.4For the following exercises, write a recursive formula for each arithmetic sequence.

Exercise 43an = {40, 60, 80, ...}

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Exercise 44 (Solution on p. 22.)

an = {17, 26, 35, ...}Exercise 45

an = {−1, 2, 5, ...}Exercise 46 (Solution on p. 22.)

an = {12, 17, 22, ...}Exercise 47

an = {−15,−7, 1, ...}Exercise 48 (Solution on p. 22.)

an = {8.9, 10.3, 11.7, ...}Exercise 49

an = {−0.52,−1.02,−1.52, ...}Exercise 50 (Solution on p. 22.)

an = { 15 , 9

20 , 710 , ...}

Exercise 51an = {− 1

2 ,− 54 ,−2, ...}

Exercise 52 (Solution on p. 22.)

an = { 16 ,− 11

12 ,−2, ...}For the following exercises, write a recursive formula for the given arithmetic sequence, and then �nd thespeci�ed term.

Exercise 53an = {7, 4, 1, ...}; Find the 17th term.

Exercise 54 (Solution on p. 22.)

an = {4, 11, 18, ...}; Find the 14th term.

Exercise 55an = {2, 6, 10, ...}; Find the 12th term.

For the following exercises, use the explicit formula to write the �rst �ve terms of the arithmetic sequence.

Exercise 56 (Solution on p. 22.)

an = 24− 4n

Exercise 57an = 1

2n− 12

For the following exercises, write an explicit formula for each arithmetic sequence.

Exercise 58 (Solution on p. 22.)

an = {3, 5, 7, ...}Exercise 59

an = {32, 24, 16, ...}Exercise 60 (Solution on p. 22.)

an = {−5, 95, 195, ...}Exercise 61

an = {−17, − 217, − 417,...}Exercise 62 (Solution on p. 22.)

an = {1.8, 3.6, 5.4, ...}Exercise 63

an = {−18.1,−16.2,−14.3, ...}

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Exercise 64 (Solution on p. 22.)

an = {15.8, 18.5, 21.2, ...}Exercise 65

an = { 13 ,− 4

3 ,−3, ...}Exercise 66 (Solution on p. 22.)

an = {0, 13 , 2

3 , ...}Exercise 67

an = {−5,− 103 ,− 5

3 , . . . }For the following exercises, �nd the number of terms in the given �nite arithmetic sequence.

Exercise 68 (Solution on p. 22.)

an = {3,− 4,− 11, ...,− 60}Exercise 69

an = {1.2, 1.4, 1.6, ..., 3.8}Exercise 70 (Solution on p. 22.)

an = { 12 , 2, 7

2 , ..., 8}

7.3 Graphical

For the following exercises, determine whether the graph shown represents an arithmetic sequence.

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Exercise 71

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Exercise 72 (Solution on p. 22.)

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For the following exercises, use the information provided to graph the �rst 5 terms of the arithmetic sequence.

Exercise 73a1 = 0, d = 4

Exercise 74 (Solution on p. 22.)

a1 = 9; an = an−1 − 10Exercise 75

an = −12 + 5n

7.4 Technology

For the following exercises, follow the steps to work with the arithmetic sequence an = 3n − 2 using agraphing calculator:

� Press [MODE]

- Select SEQ in the fourth line- Select DOT in the �fth line- Press [ENTER]

� Press [Y=]

- nMin is the �rst counting number for the sequence. Set nMin = 1- u (n) is the pattern for the sequence. Set u (n) = 3n− 2- u (nMin) is the �rst number in the sequence. Set u (nMin) = 1

� Press [2ND] then [WINDOW] to go to TBLSET

- SetTblStart = 1- Set∆Tbl = 1- Set Indpnt: Auto and Depend: Auto

� Press [2ND] then [GRAPH] to go to the TABLE

Exercise 76 (Solution on p. 23.)

What are the �rst seven terms shown in the column with the heading u (n) ?Exercise 77Use the scroll-down arrow to scroll ton = 50. What value is given for u (n) ?Exercise 78 (Solution on p. 23.)

Press [WINDOW]. SetnMin = 1, nMax = 5, xMin = 0, xMax = 6, yMin = −1, and yMax =14.Then press [GRAPH]. Graph the sequence as it appears on the graphing calculator.

For the following exercises, follow the steps given above to work with the arithmetic sequence an = 12n + 5

using a graphing calculator.

Exercise 79What are the �rst seven terms shown in the column with the headingu (n) in the TABLE feature?

Exercise 80 (Solution on p. 24.)

Graph the sequence as it appears on the graphing calculator. Be sure to adjust the WINDOWsettings as needed.

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7.5 Extensions

Exercise 81Give two examples of arithmetic sequences whose 4th terms are 9.

Exercise 82 (Solution on p. 25.)

Give two examples of arithmetic sequences whose 10th terms are 206.

Exercise 83Find the 5th term of the arithmetic sequence {9b, 5b, b, . . . }.Exercise 84 (Solution on p. 25.)

Find the 11th term of the arithmetic sequence {3a− 2b, a + 2b,−a + 6b . . . }.Exercise 85At which term does the sequence {5.4, 14.5, 23.6, ...} exceed 151?

Exercise 86 (Solution on p. 25.)

At which term does the sequence { 173 , 31

6 , 143 , ...} begin to have negative values?

Exercise 87For which terms does the �nite arithmetic sequence { 5

2 , 198 , 9

4 , ..., 18} have integer values?

Exercise 88 (Solution on p. 25.)

Write an arithmetic sequence using a recursive formula. Show the �rst 4 terms, and then �nd the31st term.

Exercise 89Write an arithmetic sequence using an explicit formula. Show the �rst 4 terms, and then �nd the28th term.

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Solutions to Exercises in this Module

Solution to Exercise (p. 3)The sequence is arithmetic. The common di�erence is −− 2.Solution to Exercise (p. 3)The sequence is not arithmetic because 3− 1 6= 6− 3.Solution to Exercise (p. 4){1, 6, 11, 16, 21}Solution to Exercise (p. 5)a2 = 2Solution to Exercise (p. 7)

a1 = 25

an = an−1 + 12, for n ≥ 2Solution to Exercise (p. 10)an = 53− 3nSolution to Exercise (p. 11)There are 11 terms in the sequence.Solution to Exercise (p. 12)The formula is Tn = 10 + 4n, and it will take her 42 minutes.Solution to Exercise (p. 13)A sequence where each successive term of the sequence increases (or decreases) by a constant value.Solution to Exercise (p. 13)We �nd whether the di�erence between all consecutive terms is the same. This is the same as saying thatthe sequence has a common di�erence.Solution to Exercise (p. 13)Both arithmetic sequences and linear functions have a constant rate of change. They are di�erent becausetheir domains are not the same; linear functions are de�ned for all real numbers, and arithmetic sequencesare de�ned for natural numbers or a subset of the natural numbers.Solution to Exercise (p. 13)The common di�erence is 1

2Solution to Exercise (p. 13)The sequence is not arithmetic because 16− 4 6= 64− 16.Solution to Exercise (p. 14)

0,23,

43, 2,

83

Solution to Exercise (p. 14)0,−5,−10,−15,−20Solution to Exercise (p. 14)a4 = 19Solution to Exercise (p. 14)a6 = 41Solution to Exercise (p. 14)a1 = 2Solution to Exercise (p. 14)a1 = 5Solution to Exercise (p. 14)a1 = 6Solution to Exercise (p. 14)a21 = −13.5

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Solution to Exercise (p. 14)−19,−20.4,−21.8,−23.2,−24.6Solution to Exercise (p. 15)

a1 = 17; an = an−1 + 9 n ≥ 2Solution to Exercise (p. 15)

a1 = 12; an = an−1 + 5 n ≥ 2Solution to Exercise (p. 15)

a1 = 8.9; an = an−1 + 1.4 n ≥ 2Solution to Exercise (p. 15)

a1 = 15 ; an = an−1 + 1

4 n ≥ 2Solution to Exercise (p. 15)

1 = 16 ; an = an−1 − 13

12 n ≥ 2Solution to Exercise (p. 15)a1 = 4; an = an−1 + 7; a14 = 95Solution to Exercise (p. 15)First �ve terms: 20, 16, 12, 8, 4.Solution to Exercise (p. 15)an = 1 + 2nSolution to Exercise (p. 15)an = −105 + 100nSolution to Exercise (p. 15)an = 1.8nSolution to Exercise (p. 15)an = 13.1 + 2.7nSolution to Exercise (p. 16)an = 1

3n− 13

Solution to Exercise (p. 16)There are 10 terms in the sequence.Solution to Exercise (p. 16)There are 6 terms in the sequence.Solution to Exercise (p. 18)The graph does not represent an arithmetic sequence.

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Solution to Exercise (p. 19)

Solu-tion to Exercise (p. 19)1, 4, 7, 10, 13, 16, 19

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Solution to Exercise (p. 19)

Solu-

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tion to Exercise (p. 19)

Solu-tion to Exercise (p. 20)Answers will vary. Examples: an = 20.6nandan = 2 + 20.4n.Solution to Exercise (p. 20)a11 = −17a + 38bSolution to Exercise (p. 20)The sequence begins to have negative values at the 13th term, a13 = − 1

3Solution to Exercise (p. 20)Answers will vary. Check to see that the sequence is arithmetic. Example: Recursive formula: a1 = 3, an =an−1 − 3. First 4 terms: 3, 0,−3,−6 a31 = −87

Glossary

De�nition 1: arithmetic sequencea sequence in which the di�erence between any two consecutive terms is a constant

De�nition 2: common di�erencethe di�erence between any two consecutive terms in an arithmetic sequence

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