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B. Using Matrices to Encrypt Messages

# B. Using Matrices to Encrypt Messages

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Section 7.5 Matrix Applications and Technology Use

695

with a fixed, invertible matrix A, we will develop a matrix B such that the product AB is

possible, and our secret message is encrypted in AB. At the receiving end, they will need

to know AϪ1 to decipher the message, since AϪ1 1AB2 ϭ 1AϪ1A2B ϭ B, which is the

original message. Note that in case an intruder were to find matrix A (perhaps purchasing the information from a disgruntled employee), we must be able to change it easily.

This means we should develop a method for generating a matrix A, with integer entries,

where A is invertible and AϪ1 also has integer entries.

EXAMPLE 3

Finding an Invertible Matrix A Where Both A and A؊1 Have Integer Entries

Find an invertible 3 ϫ 3 matrix A as just described, and its inverse AϪ1.

Solution

Begin with any 3 ϫ 3 matrix that has only 1s or Ϫ1s on its main diagonal, and 0s

below the diagonal. The upper triangle can consist of any integer values you

choose, as in

Ϫ1

£ 0

0

WORTHY OF NOTE

Performing row operations is

explained in more detail in the

graphing calculator manual

accompanying this text.

5

1

0

Ϫ1

8§.

1

Now, use any of the elementary row operations to

make the matrix more complex. For instance,

we’ll use a calculator to create a new matrix by

(1) using R1 ϩ R2 S R2 to create matrix [C], then

(2) using R1 ϩ R3 S R3 to create matrix [D], then

(3) using R3 ϩ R2 S R2 to create matrix [E], and

finally (4) Ϫ2 R1 ϩ R3 S R3 to obtain our final

matrix [A]. To begin, enter the initial matrix as

matrix [B]. For (1) R1 ϩ R2 S R2, go to the

D:row؉( and press

to bring this option to the

home screen. This feature requires us to name the

matrix we’re using, and to indicate what rows to

add, so we enter D:row؉([B], 1, 2). The screen

shown in Figure 7.27 indicates we’ve placed the

result in matrix [C]. For R1 ϩ R3 S R3, recall

D:row؉([B], 1, 2) using 2nd

and change it to

D:row؉([C], 1, 3) STO [D] (Figure 7.28). Repeat

this process for (3) R3 ϩ R2 S R2 to create matrix

[E]: D:row؉([D], 3, 2) STO [E] (Figure 7.29).

Finally, we compute (4) Ϫ2R1 ϩ R3 S R3 using

the (new) option F:*row؉(؊2, [E], 1, 3) STO [A]

and the process is complete (Figure 7.30). The

entries of matrix A are all integers, and AϪ1 exists

and also has integer entries (Figure 7.31). This will

always be the case for matrices created in this way.

Figure 7.27

Figure 7.28

ENTER

ENTER

Figure 7.30

Figure 7.29

Figure 7.31

Now try Exercises 11 through 16

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CHAPTER 7 Matrices and Matrix Applications

EXAMPLE 4

Using Matrices to Encrypt Messages

Set up a substitution cipher to encode the message MATH IS SWEET, and then use

the matrix A from Example 3 to encrypt it.

Solution

For the cipher, we will associate a unique number to every letter in the alphabet. This

can be done randomly or using a systematic approach. Here we choose to associate 0

with a blank space, and assign 1 to A, Ϫ1 to B, 2 to C, Ϫ2 to D, and so on.

Blank

A

B

C

D

E

F

G

H

I

J

K

L

M

0

1

Ϫ1

2

Ϫ2

3

Ϫ3

4

Ϫ4

5

Ϫ5

6

Ϫ6

7

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

Ϫ7

8

Ϫ8

9

Ϫ9

10 Ϫ10 11 Ϫ11 12 Ϫ12 13

Ϫ13

Now encode the secret message as shown:

M

A

7

1

T

H

Ϫ10 Ϫ4

0

I

S

5

10

0

S

W

E

E

T

10

12

3

3

Ϫ10

We next enter the coded message into a new matrix B, by entering it letter by letter

into the columns of B. Note that since the encrypting matrix A is 3 ϫ 3, B must

have 3 rows for multiplication to be possible. The result is

M

Bϭ £A

T

H

*

I

S

*

S

W

E

E

T

7

*§ ϭ £ 1

*

Ϫ10

Ϫ4

0

5

10

0

10

12

3

3

Ϫ10

0

Since the message is too short to fill matrix B, we use blank spaces to complete the

final column. Computing the product AB encrypts the message, and only someone

Ϫ1

AB ϭ £ Ϫ2

1

8

ϭ £ Ϫ73

Ϫ18

5

11

Ϫ5

Ϫ1

7

7§ £ 1

2 Ϫ10

Ϫ1

43

6

Ϫ20

50

30

0

30

3

Ϫ4

0

5

10

0

10

12

3

3

Ϫ10

0

10

20 §

Ϫ10

The encrypted message is 8, Ϫ73, Ϫ18, Ϫ1, 43, 6, Ϫ20, 50, 30, 0, 30, 3, 10, 20, Ϫ10.

Now try Exercises 17 through 22 ᮣ

EXAMPLE 5

Deciphering Encrypted Messages Using an Inverse Matrix

Decipher the encrypted message from Example 4 using AϪ1 from Example 3.

Solution

The received message is 8, Ϫ73, Ϫ18, Ϫ1, 43, 6, Ϫ20, 50, 30, 0, 30, 3, 10, 20,

Ϫ10, and is the result of the product AB. To find matrix B, we apply AϪ1 since

AϪ1 1AB2 ϭ 1AϪ1 A2B ϭ B. Writing the received message in matrix form we have

8

AB ϭ £ Ϫ73

Ϫ18

Ϫ1

43

6

Ϫ20

50

30

0

30

3

10

20 §

Ϫ10

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Section 7.5 Matrix Applications and Technology Use

Next multiply AB by AϪ1 on the left, to determine matrix B:

Ϫ1

A

Ϫ57

1AB2 ϭ £ Ϫ11

1

ϭ £

B. You’ve just seen how

we can use matrices for

encryption/decryption

7

1

Ϫ10

5

1

0

Ϫ4

0

5

Ϫ46

8

Ϫ9 § £ Ϫ73

1 Ϫ18

10

0

10

Ϫ1

43

6

Ϫ20

50

30

0

30

3

10

20 §

Ϫ10

Ϫ10

0§ ϭ B

0

12

3

3

Writing matrix B in sentence form gives 7, 1, Ϫ10, Ϫ4, 0, 5, 10, 0, 10, 12, 3, 3,

Ϫ10, 0, 0, and using the substitution cipher to replace numbers with letters, reveals

the message MATH IS SWEET.

Now try Exercises 23 through 28

7.5 EXERCISES

1. Slammin’ Drums manufactures several different

types of drums. Its most popular drums are the

22– bass drum, the 12– tom, and the 14– snare

drum. The 22– bass drum requires 7 ft2 of skin,

8.5 ft2 of wood veneer, 8 tension rods, and 11.5 ft

of hoop. The 12– tom requires 2 ft2 of skin, 3 ft2 of

wood veneer, 6 tension rods, and 6.5 ft of hoop.

The 14– snare requires 2.5 ft2 of skin, 1.5 ft2 of

wood veneer, 10 tension rods, and 7 ft of hoop. In

February, Slammin’ Drums received orders for

15 bass drums, 21 toms, and 27 snares. Use your

calculator and a matrix equation to determine how

much of each raw material they need to have on

hand to fill these orders.

2. In March, Slammin’ Drums’ orders consisted of

19 bass drums, 19 toms, and 25 snares. Use your

calculator and a matrix equation to determine how

much of each raw material they need to have on

hand to fill their orders. (See Exercise 1.)

3. The following table represents Slammin’s orders

for the months of April through July. Use your

calculator and a matrix equation to determine how

much of each raw material they need to have on

hand to fill these orders. (See Exercise 1.) (Hint:

Using a clever 4 ϫ 3 and 3 ϫ 1 matrix can reduce

this problem to a single step.)

April

May

June

July

Bass drum

23

21

17

14

Tom

20

18

15

17

Snare drum

29

35

27

25

4. The following table represents Slammin’s orders

for the months of August through November. Use

your calculator and a matrix equation to determine

how much of each raw material they need to have

on hand to fill their orders. (See Exercise 1.)

(Hint: Using a clever 4 ϫ 3 and 3 ϫ 1 matrix can

reduce this problem to a single step.)

August

September

October

November

Bass drum

17

22

16

12

Tom

15

14

13

11

Snare drum

32

28

27

21

5. Midwest Petroleum (MP) produces three types of

combustibles using common refined gasoline and

vegetable products. The first is E10 (also known as

gasohol), the second is E85, and the third is

biodiesel. One gallon of E10 requires 0.90 gal of

gasoline, 2 lb of corn, 1 oz of yeast, and 0.5 gal of

water. One gallon of E85 requires 0.15 gal of

gasoline, 17 lb of corn, 8.5 oz of yeast, and 4.25gal

of water. One gallon of biodiesel requires 20 lb of

corn and 3 gal of water. One week’s production at

MP consisted of 100,000 gal of E10, 15,000 gal of

E85, and 7000 gal of biodiesel. Use your calculator

and a matrix equation to determine how much of

each raw material they used to fill their orders.

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6. The following table represents Midwest

Petroleum’s production for the next 3 weeks. Use

your calculator and a matrix equation to determine

the total amount of raw material they need to fill

their orders (see Exercise 5).

Week 2

Week 3

Week 4

E10

110,000

95,000

105,000

E85

17,000

18,000

20,000

6,000

8,000

10,000

Biodiesel

7. Roll-X Watches makes some of the finest

wristwatches in the world. Their most popular

model is the Clam. It comes in three versions:

Silver, Gold, and Platinum. Management thinks

there might be a thief in the production line, so

they decide to closely monitor the precious metal

consumption. A Silver Clam contains 1.2 oz of

silver and 0.2 oz of gold. A Gold Clam contains

0.5 oz of silver, 0.8 oz of gold, and 0.1 oz of

platinum. A Platinum Clam contains 0.2 oz of

silver, 0.5 oz of gold, and 0.7 oz of platinum.

During the first week of monitoring, the production

team used 10.9 oz of silver, 9.2 oz of gold, and

2.3 oz of platinum. Use your calculator and a

matrix equation to determine the number of each

type of watch that should have been produced.

8. The following table contains the precious metal

consumption of the Roll-X Watch production line

during the next five weeks (see Exercise 7). Use

your graphing calculator to determine the number

of each type of watch that should have been

produced each week. For which week does the data

seem to indicate a possible theft of precious metal?

Ounces

Silver

Week 1

Week 2

Week 3

Week 4

Week 5

13.1

9

12.9

11.9

11.2

Gold

11

7.7

8.6

8.4

9.5

Platinum

2.5

1.5

0.9

2.8

1.7

9. There are three classes of grain, of which three

bundles from the first class, two from the second,

and one from the third make 39 measures. Two of

the first, three of the second, and one of the third

make 34 measures. And one of the first, two of the

second, and three of the third make 26 measures.

How many measures of grain are contained in one

bundle of each class? (This is the historic problem

from the Chiu chang suan shu.)

10. During a given week, the measures of grain that

make up the bundles in Exercise 9 can vary

slightly. Three local Chinese bakeries always buy

the same numbers of bundles, as outlined in

Exercise 9. That is to say, bakery 1 buys three

bundles of the first class, two of the second, and

one of the third. Bakery 2 buys two of the first,

three of the second, and one of the third. And

finally, bakery 3 buys one of the first, two of the

second, and three of the third. The following table

outlines how many measures of grain each bakery

received each day. How many measures of grain

were contained in one bundle of each class, on

each day?

Mon

Tues

Wed

Thurs

Fri

Bakery 1

(measures)

39

38

38

37.75

39.75

Bakery 2

(measures)

34

33

33.5

32.5

35

Bakery 3

(measures)

26

26

27

26.25

27.25

For Exercises 11–16, use the criteria indicated to find

3 ؋ 3 matrices A and A؊1, where the entries of both are

all integers.

11. The lower triangle is all zeroes.

12. The upper triangle is all zeroes.

13. a2,1 ϭ 5

14. a3,2 ϭ Ϫ2

15. a3,1 ϭ 1 and a2,3 ϭ 2

16. a2,1 ϭ Ϫ3 and a1,3 ϭ 1

17. Use the matrix A you created in Exercise 11 and

the substitution cipher from Example 4 to encrypt

18. Use the matrix A you created in Exercise 12 and

the substitution cipher from Example 4 to encrypt

19. Design your own substitution cipher. Then use it

and the matrix A you created in Exercise 13 to

encrypt the title of your favorite movie.

20. Design your own substitution cipher. Then use it

and the matrix A you created in Exercise 14 to

encrypt the title of your favorite snack food.

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Making Connections

21. Design your own substitution cipher. Then use it

and the matrix A you created in Exercise 15 to

encrypt the White House switchboard phone

number, 202-456-1414.

22. Design your own substitution cipher. Then use it

and the matrix A you created in Exercise 16 to

encrypt the Casa Rosada switchboard phone

number 54-11-4344-3600. The Casa Rosada, or

Pink House, consists of the offices of the president

of Argentina.

23. Use the matrix AϪ1 from Exercise 11, and the

appropriate substitution cipher to decrypt the

message from Exercise 17.

25. Use the matrix AϪ1 from Exercise 13, and the

appropriate substitution cipher to decrypt the

message from Exercise 19.

26. Use the matrix AϪ1 from Exercise 14, and the

appropriate substitution cipher to decrypt the

message from Exercise 20.

27. Use the matrix AϪ1 from Exercise 15, and the

appropriate substitution cipher to decrypt the

message from Exercise 21.

28. Use the matrix AϪ1 from Exercise 16, and the

appropriate substitution cipher to decrypt the

message from Exercise 22.

24. Use the matrix AϪ1 from Exercise 12, and the

appropriate substitution cipher to decrypt the

message from Exercise 18.

MAKING CONNECTIONS

Making Connections: Graphically, Symbolically, Numerically, and Verbally

Eight matrices A through H are given. Use a graphing calculator to help match the characteristics or operations

indicated in 1 through 16 to one of the eight matrices. In some cases, the response requires two matrices.

3 Ϫ2

Aϭ c

d

1 4

0

E ϭ £ Ϫ2

1

3

4

5

2

Ϫ1 §

Ϫ6

Ϫ2 3

Bϭ c

d

2 Ϫ4

4

Cϭ c

1

1 0

F ϭ £0 1

0 0

1

G ϭ £ Ϫ3

Ϫ2

0

1

3

4

7

0

Ϫ1 §

Ϫ1

9. ____ the product is c

1. ____ 3 ϫ 3, noninvertible

Ϫ2

Dϭ £ 1

0

0 Ϫ2

d

Ϫ3 5

Hϭ c

Ϫ8

Ϫ5

2. ____ determinant is 1

Ϫ3

10. ____ the product is £ 8

3

3. ____ entry a3,2 is 3

11. ____ determinant is Ϫ67

4. ____ the sum is c

1

3

1

d

0

12. ____ determinant is 0

13. ____ 3 ϫ 2 matrix

5. ____ determinant is 14

6. ____ matrix squared is c

7. ____ matrix inverse is c

8. ____ entry a3,1 is Ϫ2

10

Ϫ12

Ϫ2

Ϫ1

Ϫ18

d

22

Ϫ1.5

d

Ϫ1

14. ____ 2 ϫ 3 matrix

15. ____ augmented matrix

16. ____ identity matrix

5

3

3 Ϫ1

Ϫ5 2

14

d

8

Ϫ15

Ϫ12

Ϫ9

29

18 §

15

1 0

d

0 1

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CHAPTER 7 Matrices and Matrix Applications

SUMMARY AND CONCEPT REVIEW

Solving Linear Systems Using Matrices and Row Operations

SECTION 7.1

KEY CONCEPTS

• A matrix is a rectangular arrangement of numbers. An m ϫ n matrix has m rows and n columns.

• An augmented matrix is derived from a system of linear equations by augmenting the coefficient matrix (formed

by the variable coefficients) with the matrix of constants.

• One matrix method for solving systems of equations is by triangularizing the augmented matrix.

• An inconsistent system with no solutions will yield a contradictory statement such as 0 ϭ 1. A dependent system

with infinitely many solutions will yield an identity statement such as 0 ϭ 0.

EXERCISES

1. Write an example of the following matrices:

a. 2 ϫ 3

b. 3 ϫ 2

c. 3 ϫ 3, in triangular form

Solve by triangularizing the augmented matrix. If the system is linearly dependent, state the answer using a parameter.

Use a calculator for Exercise 5.

2. e

x Ϫ 2y ϩ 2z ϭ 7

3. • 2x ϩ 2y Ϫ z ϭ 5

3x Ϫ y ϩ z ϭ 6

x Ϫ 2y ϭ 6

4x Ϫ 3y ϭ 4

2x Ϫ y ϩ 2z ϭ Ϫ1

4. • x ϩ 2y ϩ 2z ϭ Ϫ3

3x Ϫ 4y ϩ 2z ϭ 1

2w ϩ x ϩ 2y Ϫ 3z ϭ Ϫ19

w Ϫ 2x Ϫ y ϩ 4z ϭ 15

5. μ

x ϩ 2y Ϫ z ϭ 1

3w Ϫ 2x

Ϫ 5z ϭ Ϫ60

The Algebra of Matrices

SECTION 7.2

KEY CONCEPTS

• The entries of a matrix are denoted aij, where i gives the row and j gives the column of its location.

• Two matrices A and B of equal size (or order) are equal if corresponding entries are equal.

• The sum or difference of two matrices of equal order is found by combining corresponding entries:

A ϩ B ϭ 3 aij ϩ bij 4

• The identity matrix for addition is an m ϫ n matrix whose entries are all zeroes.

• To perform scalar multiplication, take the product of the constant with each entry in the matrix, forming a new

matrix of like size. For matrix A: kA ϭ 3 kaij 4 .

Matrix

multiplication is performed as row entry ϫ column entry. For an m ϫ n matrix A ϭ 3aij 4 and an s ϫ t

matrix B ϭ 3bij 4 , AB is possible if n ϭ s. The result will be an m ϫ t matrix P ϭ 3 pij 4 , where pij is the product of

the ith row of A with the jth column of B.

• When technology is used to perform operations on matrices, carefully enter each matrix into the calculator. Then

double check that each entry is correct and appraise the results to see if they are reasonable.

EXERCISES

Compute the operations indicated below (if possible), using the following matrices.

Ϫ1

4

c Ϫ1

8

Ϫ3

4

Ϫ7 d

8

Ϫ1 3

C ϭ £ 5 Ϫ2

6 Ϫ3

8. C Ϫ B

9. 8A

Ϫ7 6

Bϭ c

d

1 Ϫ2

6. A ϩ B

7. B Ϫ A

11. C ϩ D

12. D Ϫ C

13. BC

14. Ϫ4D

4

2

2

D ϭ £ 0.5

4

10. BA

15. CD

Ϫ3

1

0.1

0

Ϫ1 §

5

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Summary and Concept Review

SECTION 7.3

701

Solving Linear Systems Using Matrix Equations

KEY CONCEPTS

• The identity matrix for multiplication I, has 1’s on the main diagonal and 0’s for all other entries. For any n ϫ n

matrix A, the identity matrix is also an n ϫ n matrix In, where AIn ϭ In A ϭ A.

• For an n ϫ n (square) matrix A, the inverse matrix for multiplication is a matrix B such that AB ϭ BA ϭ In. For

matrix A the inverse is denoted AϪ1 . Only square matrices have inverses.

• Any n ϫ n system of equations can be written as a matrix equation and solved (if a unique solution exists) using

an inverse matrix. The system

e

2x ϩ 3y ϭ 7

2

is written as c

x Ϫ 4y ϭ Ϫ2

1

3

x

7

d.

dc d ϭ c

Ϫ4 y

Ϫ2

• Every square matrix has a real number associated with it, called its determinant. For 2 ϫ 2 matrix

Aϭ c

a11 a12

d , det1A2 ϭ a11a22 Ϫ a21a12.

a21 a22

• If the determinant of a matrix is zero, the matrix is said to be singular or noninvertible. If the coefficient matrix of

a matrix equation is noninvertible, the system is either inconsistent or dependent.

EXERCISES

Complete Exercises 16 through 18 using the following matrices:

1 0

0.2 0.2

2 Ϫ1

10

d

Bϭ c

d

Cϭ c

d

Dϭ c

0 1

Ϫ0.6 0.4

3 1

Ϫ15

16. Exactly one of the matrices given is singular. Compute each determinant to identify it.

Aϭ c

Ϫ6

d

9

17. Show that AB ϭ BA ϭ B. What can you conclude about matrix A?

18. Show that BC ϭ CB ϭ I. What can you conclude about matrix C?

Use a graphing calculator to complete Exercises 19 through 21, using the matrices given:

1

E ϭ £ Ϫ2

Ϫ1

Ϫ2

1

Ϫ1

3

Ϫ5 §

Ϫ2

1

Fϭ £ 0

Ϫ2

Ϫ1

1

1

1

0 §

Ϫ1

Ϫ1

Gϭ £ 0

2

0

1

1

Ϫ1

1

19. Exactly one of the matrices is singular. Determine which one.

20. Compute the products FG and GF. What can you conclude about matrix G?

21. Verify that EG

GE and EF

FE. What can you conclude?

Solve manually using a matrix equation.

22. e

2x Ϫ 5y ϭ 14

Ϫ3y ϩ 4x ϭ Ϫ14

SECTION 7.4

Solve using a matrix equation and your calculator.

0.5x Ϫ 2.2y ϩ 3z ϭ Ϫ8

23. • Ϫ0.6x Ϫ

y ϩ 2z ϭ Ϫ7.2

x ϩ 1.5y Ϫ 0.2z ϭ 2.6

Applications of Matrices and Determinants:

Cramer’s Rule, Partial Fractions, and More

KEY CONCEPTS

• Cramer’s rule uses a ratio of determinants to solve systems of equations (if solutions exist).

a11 a12

` is a11a22 Ϫ a21a12.

• The determinant of the 2 ϫ 2 matrix `

a21 a22

• To compute the value of 3 ϫ 3 and larger determinants, a calculator is generally used.

• Determinants can be used to find the area of a triangle in the plane if the vertices of the triangle are known, and as

a test to see if three points are collinear.

• A system of equations can be used to write a rational expression as a sum of its partial fractions.

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EXERCISES

Solve using Cramer’s rule. Use a graphing calculator for Exercise 26.

2x ϩ y Ϫ z ϭ Ϫ1

5x ϩ 6y ϭ 8

24. e

25. • x Ϫ 2y ϩ z ϭ 5

10x Ϫ 2y ϭ Ϫ9

3x Ϫ y ϩ 2z ϭ 8

2x ϩ y

ϭ Ϫ2

26. • Ϫx ϩ y ϩ 5z ϭ 12

3x Ϫ 2y ϩ z ϭ Ϫ8

27. Find the area of a triangle whose vertices have the coordinates (6, 1), 1Ϫ1, Ϫ62 , and 1Ϫ6, 22.

7x2 Ϫ 5x ϩ 17

.

28. Find the partial fraction decomposition for 3

x Ϫ 2x2 ϩ 3x Ϫ 6

SECTION 7.5

Matrix Applications and Technology Use

KEY CONCEPTS

• In studies involving numerous constraints, many variables, and/or large amounts of data, matrix methods have a

distinct advantage over other solution methods.

• Square matrices with integer coefficients rarely have an inverse that also has integer coefficients. However these

can be carefully constructed.

• A matrix [A] can be used to encode a written message, with the matrix [A]Ϫ1 being used to decode it.

29. J.P. Sailing and Co. hand builds two types of classic sailing boats made entirely of wood. Both

the 13-foot Laser class dinghy and 25-foot Bermuda sloop yacht are constructed primarily with

three types of wood: Douglas fir, Brazilian jequitiba, and, of course, Asian teak. Each dingy is

built using 120 ft2 of fir, 132 ft2 of jequitiba, and 50 ft2of teak. The construction process for the

yacht uses 270 ft2 of fir, 260 ft2 of jequitiba, and 108 ft2 of teak. In their first year of production,

J.P. Sailing built 11 dinghies and 5 yachts. For the following 4 yr, their dingy production

increased by 3 per year, while the yacht production increased by 2 per year. Use your calculator

and a matrix equation to determine how much of each type of wood they used during each of

their first five years of production as well as the 5-yr totals.

30. In addition to being one of the world’s first celebrity chefs, Marie-Antoine Carême (1784 –1833)

was a master pastry chef. Had he used matrices to encode his favorite culinary math secret,

perhaps his message would have been:

11, 10, 25, Ϫ25, Ϫ6, Ϫ59, 33, 13, 76, 34, 43, 69, Ϫ3, 0, Ϫ6

1

If Carême used the cipher on page 696 (Example 4) and the matrix A ϭ £ 1

for the encoding process, decode the message to find the

2

unconventional culinary secret that is actually familiar to students

of mathematics.

2

3

4

3

7

PRACTICE TEST

Solve each system by triangularizing the augmented

matrix and using back-substitution.

1. e

3x ϩ 8y ϭ Ϫ5

x ϩ 10y ϭ 2

3x Ϫ y ϩ 5z ϭ 1

2. • 3x ϩ y ϩ 4z ϭ 4

x ϩ y ϩ z ϭ 73

4x Ϫ 5y Ϫ 6z ϭ 5

3. • 2x Ϫ 3y ϩ 3z ϭ 0

x ϩ 2y Ϫ 3z ϭ 5

4. Given matrices A and B, compute:

2

a. A Ϫ B

b. B

c. AB

d. AϪ1

5

Ϫ3 Ϫ2

3

3

Aϭ c

d

Bϭ c

d

5

4

Ϫ3 Ϫ5

e. ͿAͿ

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Practice Test

5. Given matrices C and D, use a calculator to find:

a. C Ϫ D b. Ϫ0.6D c. DC d. DϪ1 e. ͿDͿ

0.5

0

0.2

C ϭ £ 0.4 Ϫ0.5

0 §

0.1 Ϫ0.4 Ϫ0.1

2x Ϫ y ϩ z ϭ 4

• 3x Ϫ 2y ϩ 4z ϭ 9

x Ϫ 2y ϩ 8z ϭ 11

7. Solve using Cramer’s rule: e

2x Ϫ 3y ϭ 2

x Ϫ 6y ϭ Ϫ2

8. Solve using a calculator and Cramer’s rule:

2x ϩ 3y ϩ z ϭ 3

• x Ϫ 2y Ϫ z ϭ 4

x Ϫ y Ϫ 2z ϭ Ϫ1

9. Solve using a matrix equation and your calculator:

2x Ϫ 5y ϭ 11

4x ϩ 7y ϭ 4

10. Solve using a matrix equation and your calculator:

e

x Ϫ 2y ϩ 2z ϭ 7

• 2x ϩ 2y Ϫ z ϭ 5

3x Ϫ y ϩ z ϭ 6

11. Use the equality of matrices to write and solve a

system that gives values of x, y, and z so that A ϭ B.

2x ϩ y

xϩz

Bϭ c

zϪ1

2y ϩ 5

3

d and

3x ϩ 2z

3

d

yϩ 8

12. Given matrix X is a solution to AX ϭ B for the

matrix A given, find matrix B.

Xϭ £

Ϫ1

Ϫ3

2

2

§

1

A ϭ £2

3

r

3

2

10

d and A2 ϭ c

s

Ϫ3

Ϫ2

d given, find r

7

and s.

6. Use matrices to find three different solutions of the

dependent system:

Aϭ c

14. A farmer plants a triangular field with wheat. The

first vertex of the triangular field is 1 mi east and

1 mi north of his house. The second vertex is 3 mi

east and 1 mi south of his house. The third vertex is

1 mi west and 2 mi south of his house. What is the

area of the field?

15. For A ϭ c

0.5 0.1 0.2

D ϭ £ Ϫ0.1 0.1 0 §

0.3 0.4 0.8

703

Ϫ2 2

Ϫ6 3 §

4 Ϫ1

13. Use matrices and a graphing calculator to determine

which three of the following four points are collinear:

(Ϫ1, 4), (1, 3), (2, 1), (4, Ϫ1)

Create a system of equations to model each exercise,

then solve using any matrix method.

16. Dr. Brown and Dr. Stamper graduate from medical

school with \$155,000 worth of student loans. Due to

her state’s tuition reimbursement plan, Dr. Brown

owes one fourth of what Dr. Stamper owes. How

much does each doctor owe?

17. Justin is rehabbing two old houses simultaneously.

He calculates that last week he spent 23 hr working

on these houses. If he spent 8 more hours on one of

the houses, how many hours did he spend on each

house?

18. In his first month as assistant principal of

Washington High School, Mr. Johnson gave out

20 detentions. They were either for 1 day, 2 days,

or 5 days. He recorded a total of 38 days of

detention served. He also noted that there were

twice as many 2-day detentions as 5-day

detentions. How many of each type of detention

did Mr. Johnson give out?

19. The city of Cherrywood has approved a \$1,800,000

plan to renovate its historic commercial district. The

money will be coming from three separate sources.

The first is a federal program that charges a low

2% interest annually. The second is a municipal

bond offering that will cost 5% annually. The third is

a standard loan from a neighborhood bank, but it

will cost 8.5% annually. In the first year, the city will

not make any repayment on these loans and will

accrue \$94,500 more debt. The federal program and

bank loan together are responsible for \$29,500 of

this interest. How much money was originally

provided by each source?

20. Decompose the expression into partial fractions:

4x2 Ϫ 4x ϩ 3

x3 Ϫ 27

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CHAPTER 7 Matrices and Matrix Applications

CALCULATOR EXPLORATION AND DISCOVERY

Cramer’s Rule

In Section 7.4, we saw that one interesting application of matrices is Cramer’s rule. You may have noticed that when

technology is used with Cramer’s rule, the chances of making an error are fairly high, as we need to input the entries

for numerous matrices. However, as we mentioned in the chapter introduction, one of the advantages of matrices is

that they are easily programmable, and we can actually write a very simple program that will make Cramer’s rule a

more efficient method.

To begin, press the PRGM key, and then the right arrow

twice and

to create a name for our program. At the

prompt, we’ll enter CRAMER2. As we write the program, note that the needed commands (ClrHome, Disp, Pause,

Prompt, Stop) are all located in the submenus of the PRGM key, and the = sign is found under the TEST menu,

accessed using the 2nd MATH keys (the arrows “S” are used to indicate the STO key.

The following program takes the coefficients and constants of a 2 ϫ 2 linear system, and returns the ordered pair

solution in the form of x ϭ h and y ϭ k (for constants h and k). Even with minimal programming experience, reading

through the program will help you identify that Cramer’s rule is being used.

ENTER

ClrHome

Disp ''2؋2 SYSTEMS''

Pause

Disp ''AX؉BY ‫ ؍‬C''

Disp ''DX؉EY ‫ ؍‬F''

Disp ''''

Disp ''ENTER THE VALUES''

Disp ''FOR A,B,C,D,E,F''

Disp ''''

Prompt A,B,C,D,E,F

(CE–BF)/(AE–BD)SX

(AF–DC)/(AE–BD)SY

ClrHome

Disp ''THE SOLUTION IS''

Disp ''''

Disp ''X‫''؍‬

Disp X

Disp ''Y‫''؍‬

Disp Y

Stop

Exercise 1: Use the program to check the answers to Exercises 1 and 7 of the Practice Test.

Exercise 2: Create 2 ϫ 2 systems of your own that are (a) consistent, (b) inconsistent, and (c) dependent. Then verify

results using the program.

Exercise 3: Use the box on page 681 of Section 7.4 to write a similar program for 3 ϫ 3 systems. Call the program

CRAMER3, and repeat parts (a), (b), and (c) from Exercise 2.

STRENGTHENING CORE SKILLS

Augmented Matrices and Matrix Inverses

The formula for finding the inverse of a 2 ϫ 2 matrix has its roots in the more general method of computing the inverse

of an n ϫ n matrix. This involves augmenting a square matrix M with its corresponding identity In on the right (forming

an n ϫ 2n matrix), and using row operations to transform M into the identity. In some sense, as the original matrix is

transformed, the “identity part” keeps track of the operations we used to convert M and we can use the results to “get

back home,” so to speak. We’ll illustrate with the 2 ϫ 2 matrix from Section 7.3, Example 2B, where we found that

1 Ϫ2.5

6 5

6 5

c

d was the inverse matrix for c

d . We begin by augmenting c

d with the 2 ϫ 2 identity matrix.

Ϫ1

3

2 2

2 2

c

c

6

0

6

2

1

0

0

6

d Ϫ3R2 ϩ R1 S R2 c

1

0

5 1

Ϫ1 1

5 1

1 Ϫ1

0

6

d Ϫ5R2 ϩ R1 S R1 c

3

0

0

1

5

2

0

6

d Ϫ1R2 S R2 c

Ϫ3

0

1

6 Ϫ15 R1

S R1 c

d

6

0

Ϫ1

3

0

1

5 1

1 Ϫ1

0

d

3

1

Ϫ1

Ϫ2.5

d

3

As you can see, the identity is automatically transformed into the inverse matrix when this method is applied.

a b

Performing similar row operations on the general matrix c

d results in the formula given earlier.

c d

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705

Cumulative Review Chapters R–7

As you might imagine, attempting this on a general 3 ϫ 3 matrix is problematic at best, and instead we simply apply

the augmented matrix method to find AϪ1 for the 3 ϫ 3 matrix shown in blue.

2

1

£ Ϫ1 3

3 Ϫ1

Ϫ14

R2 Ϫ 7R1 S R1

→ £ 0

5R2 ϩ 7R3 S R3

0

0 1 0 0

Ϫ2 0 1 0 §

2 0 0 1

0 Ϫ4

7 Ϫ4

0 8

Ϫ14 0 0

4R3 ϩ R2 S R2

→ £ 0

7 0

4R3 ϩ R1 S R1

0

0 1

Ϫ1

To verify, we show AA

Ϫ6

1

Ϫ16

R1 ϩ 2R2 S R2→

Ϫ3R1 ϩ 2R3 S R3

2

2

10

Ϫ14

7

Ϫ7

7

Ϫ2 1.25

2

1

ϭ I: £ Ϫ1 3

3 Ϫ1

0

14

7

7 §

1.75

R3

S R3

8

R1

S R1

Ϫ14

R2

S R2

7

0

1 Ϫ0.5

Ϫ2 § £ Ϫ1

1

2

Ϫ2 1.25

2

£0

0

1

0

7 Ϫ4

Ϫ5 4

Ϫ14

£ 0

0

1 0 0

1 2 0§

Ϫ3 0 2

0 Ϫ4 Ϫ6

7 Ϫ4 1

0 1 Ϫ2

1 0 0 1

£ 0 1 0 Ϫ1

0 0 1 Ϫ2

Ϫ0.5

1

1 § ϭ £0

1.75

0

0

1

0

2

2

1.25

0

0 §

1.75

Ϫ0.5 Ϫ0.5

1

1 §.

1.25 1.75

0

0 § ✓ (AϪ1A ϭ I also checks).

1

Exercise 1: Use the preceding inverse and a matrix equation to solve the system

2x ϩ y ϭ Ϫ2

• Ϫx ϩ 3y Ϫ 2z ϭ Ϫ15 .

3x Ϫ y ϩ 2z ϭ 9

CUMULATIVE REVIEW CHAPTERS R–7

1. Perform the operations indicated.

a. 13 Ϫ 2i212 ϩ i2

b. 15 Ϫ 3i2 2

8Ϫi

c.

2ϩi

d. i 49

2. Solve S ϭ 2␲rh ϩ 2␲r2 for h.

Solve the following equations. Verify solutions using a

graphing calculator.

3. 2x Ϫ 413x ϩ 12 ϭ 5 Ϫ 4x

xϩ6

1

12

Ϫ ϭ 2

x

xϩ2

x ϩ 2x

2

5. 9x ϩ 1 ϭ 6x

4.

8. Find an equation of the line

perpendicular to the line

shown, with the same

y-intercept.

y

5

4

3

2

1

9. Given f 1x2 ϭ 3 Ϫ 4x Ϫ x2

and g1x2 ϭ 0 4 Ϫ 1x ϩ 2 0 ,

find

a. f (3)

b. g(23)

c. f 1Ϫ22

d. g1Ϫ32

Ϫ5Ϫ4Ϫ3Ϫ2Ϫ1

Ϫ1

Ϫ2

Ϫ3

Ϫ4

Ϫ5

1 2 3 4 5 x

Graph the following by using transformations of the

parent function.

1

10. y ϭ 1x Ϫ 32 2

11. y ϭ ͿxͿ Ϫ 3

2

12. y ϭ Ϫ1x ϩ 22 3

13. y ϭ 2Ϫx Ϫ 2

6. 12x ϩ 11 Ϫ x ϭ 6

in interval notation.

7. Find an equation of the line that passes through the

points (2, Ϫ2) and (Ϫ3, 5).

14. 21x Ϫ 22 ϩ 3 Յ 8

15. x2 ϩ 5 7 6x

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