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Intermediate Algebra 2e

12.2 Arithmetic Sequences

Intermediate Algebra 2e12.2 Arithmetic Sequences
  1. Preface
  2. 1 Foundations
    1. Introduction
    2. 1.1 Use the Language of Algebra
    3. 1.2 Integers
    4. 1.3 Fractions
    5. 1.4 Decimals
    6. 1.5 Properties of Real Numbers
    7. Key Terms
    8. Key Concepts
    9. Exercises
      1. Review Exercises
      2. Practice Test
  3. 2 Solving Linear Equations
    1. Introduction
    2. 2.1 Use a General Strategy to Solve Linear Equations
    3. 2.2 Use a Problem Solving Strategy
    4. 2.3 Solve a Formula for a Specific Variable
    5. 2.4 Solve Mixture and Uniform Motion Applications
    6. 2.5 Solve Linear Inequalities
    7. 2.6 Solve Compound Inequalities
    8. 2.7 Solve Absolute Value Inequalities
    9. Key Terms
    10. Key Concepts
    11. Exercises
      1. Review Exercises
      2. Practice Test
  4. 3 Graphs and Functions
    1. Introduction
    2. 3.1 Graph Linear Equations in Two Variables
    3. 3.2 Slope of a Line
    4. 3.3 Find the Equation of a Line
    5. 3.4 Graph Linear Inequalities in Two Variables
    6. 3.5 Relations and Functions
    7. 3.6 Graphs of Functions
    8. Key Terms
    9. Key Concepts
    10. Exercises
      1. Review Exercises
      2. Practice Test
  5. 4 Systems of Linear Equations
    1. Introduction
    2. 4.1 Solve Systems of Linear Equations with Two Variables
    3. 4.2 Solve Applications with Systems of Equations
    4. 4.3 Solve Mixture Applications with Systems of Equations
    5. 4.4 Solve Systems of Equations with Three Variables
    6. 4.5 Solve Systems of Equations Using Matrices
    7. 4.6 Solve Systems of Equations Using Determinants
    8. 4.7 Graphing Systems of Linear Inequalities
    9. Key Terms
    10. Key Concepts
    11. Exercises
      1. Review Exercises
      2. Practice Test
  6. 5 Polynomials and Polynomial Functions
    1. Introduction
    2. 5.1 Add and Subtract Polynomials
    3. 5.2 Properties of Exponents and Scientific Notation
    4. 5.3 Multiply Polynomials
    5. 5.4 Dividing Polynomials
    6. Key Terms
    7. Key Concepts
    8. Exercises
      1. Review Exercises
      2. Practice Test
  7. 6 Factoring
    1. Introduction to Factoring
    2. 6.1 Greatest Common Factor and Factor by Grouping
    3. 6.2 Factor Trinomials
    4. 6.3 Factor Special Products
    5. 6.4 General Strategy for Factoring Polynomials
    6. 6.5 Polynomial Equations
    7. Key Terms
    8. Key Concepts
    9. Exercises
      1. Review Exercises
      2. Practice Test
  8. 7 Rational Expressions and Functions
    1. Introduction
    2. 7.1 Multiply and Divide Rational Expressions
    3. 7.2 Add and Subtract Rational Expressions
    4. 7.3 Simplify Complex Rational Expressions
    5. 7.4 Solve Rational Equations
    6. 7.5 Solve Applications with Rational Equations
    7. 7.6 Solve Rational Inequalities
    8. Key Terms
    9. Key Concepts
    10. Exercises
      1. Review Exercises
      2. Practice Test
  9. 8 Roots and Radicals
    1. Introduction
    2. 8.1 Simplify Expressions with Roots
    3. 8.2 Simplify Radical Expressions
    4. 8.3 Simplify Rational Exponents
    5. 8.4 Add, Subtract, and Multiply Radical Expressions
    6. 8.5 Divide Radical Expressions
    7. 8.6 Solve Radical Equations
    8. 8.7 Use Radicals in Functions
    9. 8.8 Use the Complex Number System
    10. Key Terms
    11. Key Concepts
    12. Exercises
      1. Review Exercises
      2. Practice Test
  10. 9 Quadratic Equations and Functions
    1. Introduction
    2. 9.1 Solve Quadratic Equations Using the Square Root Property
    3. 9.2 Solve Quadratic Equations by Completing the Square
    4. 9.3 Solve Quadratic Equations Using the Quadratic Formula
    5. 9.4 Solve Quadratic Equations in Quadratic Form
    6. 9.5 Solve Applications of Quadratic Equations
    7. 9.6 Graph Quadratic Functions Using Properties
    8. 9.7 Graph Quadratic Functions Using Transformations
    9. 9.8 Solve Quadratic Inequalities
    10. Key Terms
    11. Key Concepts
    12. Exercises
      1. Review Exercises
      2. Practice Test
  11. 10 Exponential and Logarithmic Functions
    1. Introduction
    2. 10.1 Finding Composite and Inverse Functions
    3. 10.2 Evaluate and Graph Exponential Functions
    4. 10.3 Evaluate and Graph Logarithmic Functions
    5. 10.4 Use the Properties of Logarithms
    6. 10.5 Solve Exponential and Logarithmic Equations
    7. Key Terms
    8. Key Concepts
    9. Exercises
      1. Review Exercises
      2. Practice Test
  12. 11 Conics
    1. Introduction
    2. 11.1 Distance and Midpoint Formulas; Circles
    3. 11.2 Parabolas
    4. 11.3 Ellipses
    5. 11.4 Hyperbolas
    6. 11.5 Solve Systems of Nonlinear Equations
    7. Key Terms
    8. Key Concepts
    9. Exercises
      1. Review Exercises
      2. Practice Test
  13. 12 Sequences, Series and Binomial Theorem
    1. Introduction
    2. 12.1 Sequences
    3. 12.2 Arithmetic Sequences
    4. 12.3 Geometric Sequences and Series
    5. 12.4 Binomial Theorem
    6. Key Terms
    7. Key Concepts
    8. Exercises
      1. Review Exercises
      2. Practice Test
  14. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
    10. Chapter 10
    11. Chapter 11
    12. Chapter 12
  15. Index

Learning Objectives

By the end of this section, you will be able to:

  • Determine if a sequence is arithmetic
  • Find the general term (nnth term) of an arithmetic sequence
  • Find the sum of the first nn terms of an arithmetic sequence
Be Prepared 12.4

Before you get started, take this readiness quiz.

Evaluate 4n14n1 for the integers 1, 2, 3, and 4.
If you missed this problem, review Example 1.6.

Be Prepared 12.5

Solve the system of equations: {x+y=73x+4y=23.{x+y=73x+4y=23.
If you missed this problem, review Example 4.9.

Be Prepared 12.6

If f(n)=n2(3n+5),f(n)=n2(3n+5), find f(1)+f(20).f(1)+f(20).
If you missed this problem, review Example 3.49.

Determine if a Sequence is Arithmetic

The last section introduced sequences and now we will look at two specific types of sequences that each have special properties. In this section we will look at arithmetic sequences and in the next section, geometric sequences.

An arithmetic sequence is a sequence where the difference between consecutive terms is constant. The difference between consecutive terms in an arithmetic sequence, anan1,anan1, is d, the common difference, for n greater than or equal to two.

Arithmetic Sequence

An arithmetic sequence is a sequence where the difference between consecutive terms is always the same.

The difference between consecutive terms, anan1,anan1, is d, the common difference, for n greater than or equal to two.

This figure has two rows and three columns. The first row reads “7”, “10”,”13”, “16”, “19”, “22”, and an ellipsis, “10 minus 7, divided by 3”, “13 minus 10, divided by 3”, “16 minus 13, divided by 3”, nth term equals nth term minus 1 divided by d”

In each of these sequences, the difference between consecutive terms is constant, and so the sequence is arithmetic.

Example 12.13

Determine if each sequence is arithmetic. If so, indicate the common difference.

5,9,13,17,21,25,5,9,13,17,21,25,

4,9,12,17,20,25,4,9,12,17,20,25,

10,3,−4,−11,−18,−25,10,3,−4,−11,−18,−25,

Try It 12.25

Determine if each sequence is arithmetic. If so, indicate the common difference.

9,20,31,42,53,64,9,20,31,42,53,64, 12,6,0,−6,−12,−18,12,6,0,−6,−12,−18, 7,1,10,4,13,7,7,1,10,4,13,7,

Try It 12.26

Determine if each sequence is arithmetic. If so, indicate the common difference.

−4,4,2,10,8,16,−4,4,2,10,8,16, −3,−1,1,3,5,7,−3,−1,1,3,5,7, 7,2,−3,−8,−13,−18,7,2,−3,−8,−13,−18,

If we know the first term, a1,a1, and the common difference, d, we can list a finite number of terms of the sequence.

Example 12.14

Write the first five terms of the sequence where the first term is 5 and the common difference is d=−6.d=−6.

Try It 12.27

Write the first five terms of the sequence where the first term is 7 and the common difference is d=−4.d=−4.

Try It 12.28

Write the first five terms of the sequence where the first term is 11 and the common difference is d=−8.d=−8.

Find the General Term (nth Term) of an Arithmetic Sequence

Just as we found a formula for the general term of a sequence, we can also find a formula for the general term of an arithmetic sequence.

Let’s write the first few terms of a sequence where the first term is a1a1 and the common difference is d. We will then look for a pattern.

As we look for a pattern we see that each term starts with a1a1.

This figures shows an image of a sequence.

The first term adds 0d to the a1a1, the second term adds 1d, the third term adds 2d, the fourth term adds 3d, and the fifth term adds 4d. The number of ds that were added to a1a1 is one less than the number of the term. This leads us to the following

an=a1+(n1)dan=a1+(n1)d

General Term (nth term) of an Arithmetic Sequence

The general term of an arithmetic sequence with first term a1a1 and the common difference d is

an=a1+(n1)dan=a1+(n1)d

We will use this formula in the next example to find the 15th term of a sequence.

Example 12.15

Find the fifteenth term of a sequence where the first term is 3 and the common difference is 6.

Try It 12.29

Find the twenty-seventh term of a sequence where the first term is 7 and the common difference is 9.

Try It 12.30

Find the eighteenth term of a sequence where the first term is 13 and the common difference is −7−7.

Sometimes we do not know the first term and we must use other given information to find it before we find the requested term.

Example 12.16

Find the twelfth term of a sequence where the seventh term is 10 and the common difference is −2−2. Give the formula for the general term.

Try It 12.31

Find the eleventh term of a sequence where the ninth term is 8 and the common difference is −3.−3. Give the formula for the general term.

Try It 12.32

Find the nineteenth term of a sequence where the fifth term is 1 and the common difference is −4.−4. Give the formula for the general term.

Sometimes the information given leads us to two equations in two unknowns. We then use our methods for solving systems of equations to find the values needed.

Example 12.17

Find the first term and common difference of a sequence where the fifth term is 19 and the eleventh term is 37. Give the formula for the general term.

Try It 12.33

Find the first term and common difference of a sequence where the fourth term is 17 and the thirteenth term is 53. Give the formula for the general term.

Try It 12.34

Find the first term and common difference of a sequence where the third term is 2 and the twelfth term is −25.−25. Give the formula for the general term.

Find the Sum of the First n Terms of an Arithmetic Sequence

As with the general sequences, it is often useful to find the sum of an arithmetic sequence. The sum, Sn,Sn, of the first nn terms of any arithmetic sequence is written as Sn=a1+a2+a3+...+an.Sn=a1+a2+a3+...+an. To find the sum by merely adding all the terms can be tedious. So we can also develop a formula to find the sum of a sequence using the first and last term of the sequence.

We can develop this new formula by first writing the sum by starting with the first term, a1,a1, and keep adding a d to get the next term as:

Sn=a1+(a1+d)+(a1+2d)++an.Sn=a1+(a1+d)+(a1+2d)++an.

We can also reverse the order of the terms and write the sum by starting with anan and keep subtracting d to get the next term as

Sn=an+(and)+(an2d)++a1.Sn=an+(and)+(an2d)++a1.

If we add these two expressions for the sum of the first n terms of an arithmetic sequence, we can derive a formula for the sum of the first n terms of any arithmetic series.

Sn=a1+(a1+d)+(a1+2d)++an+Sn=an+(and)+(an2d)++a1_________________________________________________________2Sn=(a1+an)+(a1+an)+(a1+an)++(a1+an)Sn=a1+(a1+d)+(a1+2d)++an+Sn=an+(and)+(an2d)++a1_________________________________________________________2Sn=(a1+an)+(a1+an)+(a1+an)++(a1+an)

Because there are n sums of (a1+an)(a1+an) on the right side of the equation, we rewrite the right side as n(a1+an).n(a1+an).

2Sn=n(a1+an)2Sn=n(a1+an)

We divide by two to solve for Sn.Sn.

Sn=n2(a1+an)Sn=n2(a1+an)

This give us a general formula for the sum of the first n terms of an arithmetic sequence.

Sum of the First n Terms of an Arithmetic Sequence

The sum, Sn,Sn, of the first n terms of an arithmetic sequence is

Sn=n2(a1+an)Sn=n2(a1+an)

where a1a1 is the first term and anan is the nth term.

We apply this formula in the next example where the first few terms of the sequence are given.

Example 12.18

Find the sum of the first 30 terms of the arithmetic sequence: 8, 13, 18, 23, 28, …

Try It 12.35

Find the sum of the first 30 terms of the arithmetic sequence: 5, 9, 13, 17, 21, …

Try It 12.36

Find the sum of the first 30 terms of the arithmetic sequence: 7, 10, 13, 16, 19, …

In the next example, we are given the general term for the sequence and are asked to find the sum of the first 50 terms.

Example 12.19

Find the sum of the first 50 terms of the arithmetic sequence whose general term is an=3n4.an=3n4.

Try It 12.37

Find the sum of the first 50 terms of the arithmetic sequence whose general term is an=2n5.an=2n5.

Try It 12.38

Find the sum of the first 50 terms of the arithmetic sequence whose general term is an=4n+3.an=4n+3.

In the next example we are given the sum in summation notation. To add all the terms would be tedious, so we extract the information needed to use the formula to find the sum of the first n terms.

Example 12.20

Find the sum: i=125(4i+7).i=125(4i+7).

Try It 12.39

Find the sum: i=130(6i4).i=130(6i4).

Try It 12.40

Find the sum: i=135(5i3).i=135(5i3).

Media Access Additional Online Resources

Access these online resources for additional instruction and practice with arithmetic sequences

Section 12.2 Exercises

Practice Makes Perfect

Determine if a Sequence is Arithmetic

In the following exercises, determine if each sequence is arithmetic, and if so, indicate the common difference.

77.

4,12,20,28,36,44,4,12,20,28,36,44,

78.

−7,−2,3,8,13,18,−7,−2,3,8,13,18,

79.

−15,−16,3,12,21,30,−15,−16,3,12,21,30,

80.

11,5,−1,−713,−19,11,5,−1,−713,−19,

81.

8,5,2,−1,−4,−7,8,5,2,−1,−4,−7,

82.

15,5,−5,−15,−25,−35,15,5,−5,−15,−25,−35,

In the following exercises, write the first five terms of each sequence with the given first term and common difference.

83.

a1=11a1=11 and d=7d=7

84.

a1=18a1=18 and d=9d=9

85.

a1=−7a1=−7 and d=4d=4

86.

a1=−8a1=−8 and d=5d=5

87.

a1=14a1=14 and d=−9d=−9

88.

a1=−3a1=−3 and d=−3d=−3

Find the General Term (nth Term) of an Arithmetic Sequence

In the following exercises, find the term described using the information provided.

89.

Find the twenty-first term of a sequence where the first term is three and the common difference is eight.

90.

Find the twenty-third term of a sequence where the first term is six and the common difference is four.

91.

Find the thirtieth term of a sequence where the first term is −14−14 and the common difference is five.

92.

Find the fortieth term of a sequence where the first term is −19−19 and the common difference is seven.

93.

Find the sixteenth term of a sequence where the first term is 11 and the common difference is −6.−6.

94.

Find the fourteenth term of a sequence where the first term is eight and the common difference is −3.−3.

95.

Find the twentieth term of a sequence where the fifth term is −4−4 and the common difference is −2.−2. Give the formula for the general term.

96.

Find the thirteenth term of a sequence where the sixth term is −1−1 and the common difference is −4.−4. Give the formula for the general term.

97.

Find the eleventh term of a sequence where the third term is 19 and the common difference is five. Give the formula for the general term.

98.

Find the fifteenth term of a sequence where the tenth term is 17 and the common difference is seven. Give the formula for the general term.

99.

Find the eighth term of a sequence where the seventh term is −8−8 and the common difference is −5.−5. Give the formula for the general term.

100.

Find the fifteenth term of a sequence where the tenth term is −11−11 and the common difference is −3.−3. Give the formula for the general term.

In the following exercises, find the first term and common difference of the sequence with the given terms. Give the formula for the general term.

101.

The second term is 14 and the thirteenth term is 47.

102.

The third term is 18 and the fourteenth term is 73.

103.

The second term is 13 and the tenth term is −51.−51.

104.

The third term is four and the tenth term is −38−38.

105.

The fourth term is −6−6 and the fifteenth term is 27.

106.

The third term is −13−13 and the seventeenth term is 15.

Find the Sum of the First n Terms of an Arithmetic Sequence

In the following exercises, find the sum of the first 30 terms of each arithmetic sequence.

107.

11,14,17,20,23,11,14,17,20,23,

108.

12,18,24,30,36,12,18,24,30,36,

109.

8,5,2,−1,−4,8,5,2,−1,−4,

110.

16,10,4,−2,−8,16,10,4,−2,−8,

111.

−17,−15,−13,−11,−9,−17,−15,−13,−11,−9,

112.

−15,−12,−9,−6,−3,−15,−12,−9,−6,−3,

In the following exercises, find the sum of the first 50 terms of the arithmetic sequence whose general term is given.

113.

an=5n1an=5n1

114.

an=2n+7an=2n+7

115.

an=−3n+5an=−3n+5

116.

an=−4n+3an=−4n+3

In the following exercises, find each sum.

117.

i=140(8i7)i=140(8i7)

118.

i=145(7i5)i=145(7i5)

119.

i=150(3i+6)i=150(3i+6)

120.

i=125(4i+3)i=125(4i+3)

121.

i=135(−6i2)i=135(−6i2)

122.

i=130(−5i+1)i=130(−5i+1)

Writing Exercises

123.

In your own words, explain how to determine whether a sequence is arithmetic.

124.

In your own words, explain how the first two terms are used to find the tenth term. Show an example to illustrate your explanation.

125.

In your own words, explain how to find the general term of an arithmetic sequence.

126.

In your own words, explain how to find the sum of the first nn terms of an arithmetic sequence without adding all the terms.

Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This figure shows a chart with four rows and four columns. The first row is the header row and reads, “I can”, “Confidently”, “With some help”, and “No, I don’t get it!” The column, beginning with second row reads 1. Determine if a Sequence is Arithmetic, 2. Find the General Term (nth term) of Arithmetic Sequence, and 3. Find the sum of the first Terms of an Arithmetic Sequence”. The remaining columns are blank.

After reviewing this checklist, what will you do to become confident for all objectives?

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