Chapter 3 Matrices.pdf

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C H A P T E R

3Matrices

Objectives

To be able to identify when two matrices are equal

To be able to add and subtract matrices of the same dimensions

To be able to perform multiplication of a matrix and a scalar

To be able to identify when the multiplication of two given matrices is possible

To be able to perform multiplication on two suitable matrices

To be able to find the inverse of a 2 × 2 matrix

To be able to find the determinant of a matrix

To be able to solve linear simultaneous equations in two unknowns using an

inverse matrix

3.1 Introduction to matricesA matrix is a rectangular array of numbers. The numbers in the array are called the entries in

the matrix.

The following are examples of matrices:

−1 2

−3 4

5 6

[2 1 5 6]

√ 2 3

0 0 1√ 2 0

[5]

Matrices vary in size. The size, or dimension, of the matrix is described by specifying the

number of rows (horizontal lines) and columns (vertical lines) that occur in the matrix.

The dimensions of the above matrices are, in order:

3× 2, 1× 4, 3× 3, 1 × 1.

The first number represents the number of rows and the second, the number of columns.

66

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© Michael Evans et al. 2011 Cambridge University Press



Chapter 3 — Matrices 67

Example 1

Write down the dimensions of the following matrices.

a

1 1 22 1 0

b

1

2

3

4

c

2 2 3

Solution

a 2× 3 b 4 × 1 c 1× 3

The use of matrices to store information is demonstrated by the following two examples.

Four exporters A, B, C and D sell televisions (t ), CD players (c), refrigerators (r ) and

washing machines (w). The sales in a particular month can be represented by a 4× 4 array of

numbers. This array of numbers is called a matrix.r c w t

A

B

C

D

120 95 370 250

430 380 1000 900

60 50 150 100

200 100 470 50

row 1

row 2

row 3

row 4

column 1 column 2 column 3 column 4

From the matrix it can be seen that:

exporter A sold 120 refrigerators, 95 CD players, 370 washing machines and 250

televisions

exporter B sold 430 refrigerators, 380 CD players, 1000 washing machines and 900

televisions.

The entries for the sales of refrigerators are made in column 1.

The entries for the sales of exporter A are made in row 1.

The diagram on the right represents a section of a road map.

The number of direct connecting roads between towns can be

represented in matrix form.

A B C D

A

B

C

D

0 2 1 1

2 0 1 0

1 1 0 0

1 0 0 0

B

A C

D

If A is a matrix, aij will be used to denote the entry that occurs in row i and column j of A.

Thus a 3× 4 matrix may be written:

A =

a11 a12 a13 a14

a21 a22 a23 a24

a31 a32 a33 a34

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68 Essential Mathematical Methods 1 & 2 CAS

For B, an m × n matrix may be written:

B =

b11 b12 . . . . . b1n

b21 b22 . . . . . b2n

. .

. .

. .

. .

. .

bm1 bm2 . . . . . bmn

Matrices provide a format for the storage of data. In this form the data is easily operated on.

Some calculators have a built-in facility to operate on matrices and there are computer

packages which allow the manipulation of data in matrix form.

A car dealer sells three models of a certain make and his business operates through two

showrooms. Each month he summarises the number of each model sold by a sales

matrix S:

S =

s11 s12 s13

s21 s22 s23

where si j is the number of cars of model j sold by showroom i.

So, for example, s12 is the number of sales made by showroom 1, of model 2.

If in January, showroom 1 sold three, six and two cars of models 1, 2 and 3 respectively, and

showroom 2 sold four, two and one car(s) of models 1, 2 and 3 (in that order), the sales matrix

for January would be:

S =

3 6 2

4 2 1

A matrix is, then, a way of recording a set of numbers, arranged in a particular way. As in

Cartesian coordinates, the order of the numbers is significant, so that although the matrices1 2

3 4

and

3 4

1 2

have the same numbers and the same number of elements, they are different matrices (just as(2, 1), (1, 2) are coordinates of different points).

Two matrices A, B are equal, and can be written as A = B when:

each has the same number of rows and the same number of columns

they have the same number or element at corresponding positions.

For example,

2 1 −1

0 1 3

=

1+ 1 1 −1

1− 1 1 6

2

.

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Example 2

If matrices A and B are equal, find the values of x and y.

A= 2 1

x 4 B

= 2 1

−3 y

Solution

x = −3 and y = 4

Although a matrix is made from a set of numbers, it is important to think of a matrix as a

single entity, somewhat like a ‘super number’.

Example 3

There are four rows of seats of three seats each in a minibus. If 0 is used to indicate a seat is

vacant and 1 is used to indicate a seat is occupied, write down a matrix that represents the

following:

a The 1st and 3rd rows are occupied but the 2nd and 4th rows are vacant.

b Only the seat on the front left corner of the bus is occupied.

Solution

a

1 1 1

0 0 0

1 1 1

0 0 0

b

1 0 0

0 0 0

0 0 0

0 0 0

Example 4

There are four clubs in a local football league.

Team A has 2 senior teams and 3 junior teams.

Team B has 2 senior teams and 4 junior teams.

Team C has 1 senior team and 2 junior teams.

Team D has 3 senior teams and 3 junior teams.

Represent this information in a matrix.

Solution

2 3

2 4

1 2

3 3

Note: Rows represent teams A, B, C, D and columns represent the number of senior and junior teams respectively.

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Exercise 3A

1 Write down the dimensions of the following matrices.Example 1

a 1 2

3 4 b 2 1 −1

0 1 3 c [a b c d ] d

p

q

r

s

2 There are 25 seats arranged in five rows and five columns. If 0, 1 respectively are used toExample 3

indicate whether a seat is vacant or occupied, write down a matrix that represents the

situation when:

a only seats on the two diagonals are occupied

b all seats are occupied.

3 If seating arrangements (as in Question 2) are represented by matrices, consider the matrix

in which the i, j element is 1 if i = j , but 0 if i = j . What seating arrangement does this

matrix represent?

4 At a certain school there are 200 girls and 110 boys in Year 7, 180 girls and 117 boys inExample 4

Year 8, 135 and 98 respectively in Year 9, 110 and 89 in Year 10, 56 and 53 in Year 11 and

28 and 33 in Year 12. Summarise this information in matrix form.

5 From the following, select those pairs of matrices that could be equal, and write down theExample 2

values of x, y which would make them equal.

a

32

,

0 x

, [0 x ], [0 4 ]

b

4 7

1 −2

,

1 −2

4 x

,

x 7

1 −2

, [4 x 1 −2]

c

2 x 4

−1 10 3

,

y 0 4

−1 10 3

,

2 0 4

−1 10 3

6 In each of the following find the values of the pronumerals so that matrices A and B are

equal.

a A =

2 1 −1

0 1 3

B =

x 1 −1

0 1 y

b A =

x2

B =

3 y

c A = [−3 x] B = [ y 4] d A =

1 y

4 3

B =

1 −2

4 x

7 A section of a road map connecting towns A, B, C

and D is shown. Construct the 4 × 4 matrix that

shows the number of connecting roads between

each pair of towns.

B

D

A C

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8 The statistics for the five members of a basketball team are recorded as follows.

Player A: points 21, rebounds 5, assists 5

Player B: points 8, rebounds 2, assists 3

Player C: points 4, rebounds 1, assists 1

Player D: points 14, rebounds 8, assists 60

Player E: points 0, rebounds 1, assists 2Express this data in a 5 × 3 matrix.

3.2 Addition, subtraction and multiplicationby a scalarAddition will be defined for two matrices only when they have the same number of rows and

the same number of columns. In this case the sum of two matrices is found by adding

corresponding elements. For example,

1 00 2

+

0 −34 1

=

1 −34 3

and

a11 a12

a21 a22

a31 a32

+

b11 b12

b21 b22

b31 b32

=

a11 + b11 a12 + b12

a21 + b21 a22 + b22

a31 + b31 a32 + b32

Subtraction is defined in a similar way. When the two matrices have the same number of rows

and the same number of columns the difference is found by subtracting corresponding

elements.

Example 5

Find:

a

1 0

2 0

−

2 −1

−4 1

b

2 3

−1 4

−

2 3

−1 4

Solution

a

1 0

2 0

−

2 −1

−4 1

=−1 1

6

−1

b

2 3

−1 4

−

2 3

−1 4

=

0 0

0 0

It is useful to define multiplication of a matrix by a real number. If A is an m × n matrix,

and k is a real number, then k A is an m × n matrix whose elements are k times the

corresponding elements of A. Thus:

3

2 −2

0 1

=

6 −6

0 3

These definitions have the helpful consequence that if a matrix is added to itself, the result is

twice the matrix, i.e. A +A = 2A. Similarly the sum of n matrices each equal to A is nA

(where n is a natural number).The m × n matrix with all elements equal to zero is called the zero matrix.

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Example 6

Let X =

2

4

, Y =

3

6

, A =

2 0

−1 2

, B =

5 0

−2 4

Find X + Y, 2X, 4Y+ X, X− Y,−3A,−3A+ B.

Solution

X+Y =

2

4

+

3

6

=

5

10

2X = 2

2

4

=

4

8

4Y+X = 4

36

+ 24

= 1224

+ 24

= 1428

X− Y =

2

4

−

3

6

=−1

−2

−3A = −3

2 0

−1 2

=−6 0

3 −6

−3A+ B =−6 0

3

−6

+

5 0

−2 4

=−1 0

1

−2

Example 7

If A =

3 2

−1 1

and B =

0 −4

−2 8

, find matrix X such that 2A+ X = B.

Solution

If 2A+ X = B, then X = B− 2A

∴ X =

0 −4

−2 8

− 2×

3 2

−1 1

=

0− 2× 3 −4− 2 × 2

−2− 2 ×−1 8− 2 × 1

=−6 −8

0 6

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Using the TI-Nspire2 × 2 matrices are most easily entered using

the 2 × 2 matrix template, ( +on the Clickpad), as shown.

Notice that there is also a template for

entering m by n matrices.

The matrix template can also be obtained

using +b>Math Templates

To enter the matrix A =

3 6

6 7

, use the

touchpad arrows to move between the entries

of the 2 × 2 matrix template and store

( ) the matrix as a .

Define the matrix B =

3 6

5 6.5

in a

similar way.

Entering matrices directlyTo enter matrix A without using the template,

enter the matrix row by row as [[3,6][6,7]]

and store ( ) the matrix as a .

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Addition, subtraction and multiplication by a scalarOnce A and B are defined as above, A + B,

A− B and k A can easily be determined.

Using the Casio ClassPadMatrices are accessed in the menu. Turn on

the keyboard, select the 2D tab and click CALC

in the bottom left to get the screen shown here.

Click to produce a square matrix or one

of the buttons to the left to create a row or

column matrix. To expand the 2 × 2 matrix

to a 3× 3 matrix, etc., click the again.

Click into each of the entry boxes to enter the

matrix values. The matrix can be stored as a

variable for later use in operations by clicking

and selecting a variable name

(usually a capital letter).

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Once A and B are defined as above, A+ B, A− B, k A can be determined using the

VAR keyboard to enter the operations.

Note: Variables A and B are now fixed with these matrix values unless you store

another value to that variable name or clear the variables.

Exercise 3B

1 Let X =

1

−2

, Y =

3

0

, A =

1 −1

2 3

, B =

4 0

−1 2

Find X + Y, 2X, 4Y+ X, X− Y,−3A and −3A+ B.

Example 6

2 Each showroom of a car dealer sells exactly twice as many cars of each model in February

as in January. (See example in section 3.1.)

a Given that the sales matrix for January is

3 6 2

4 2 1

, write down the sales matrix for

February.

b If the sales matrices for January and March (with twice as many cars of each model

sold in February as January) had been

1 0 0

4 2 3

and

2 1 0

6 1 4

respectively, find the

sales matrix for the first quarter of the year.

c Find a matrix to represent the average monthly sales for the first three months.

3 Let A = 1 −

1

0 2

.

Find 2A, −3A and −6A.

4 A, B, C are m × n matrices. Is it true that:

a A+ B = B+ A? b (A+ B) + C = A+ (B+ C)?

5 A =

3 2

−2 −2

and B =

0 −3

4 1

Calculate:

a 2A b 3B c 2A + 3B d 3B – 2A

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6 P =

1 0

0 3

, Q =

−1 1

2 0

, R =

0 4

1 1

Calculate:

a P+Q b P+ 3Q c 2P−Q+ R

7 If A =

3 1

−1 4

and B =

0 −10

−2 17

, find matrices X and Y such thatExample 7

2A− 3X = B and 3A+ 2Y = 2B.

8 Matrices X and Y show the production of four models a, b, c, d at two automobile factories

P , Q in successive weeks.

X = P

Q

a b c d 150 90 100 50

100 0 75 0

Y = P

Q

a b c d 160 90 120 40

100 0 50 0

week 1 week 2

Find X + Y and write what this sum represents.

3.3 Multiplication of matricesMultiplication of a matrix by a real number has been discussed in the previous section. The

definition for multiplication of matrices is less natural. The procedure for multiplying two

2× 2 matrices is shown first.

Let A= 1 3

4 2 and B

= 5 1

6 3

Then AB =

1 3

4 2

5 1

6 3

=

1× 5+ 3× 6 1× 1 + 3 × 3

4× 5 + 2 × 6 4 × 1+ 2 × 3

=

23 10

32 10

and BA =

5 16 3

1 34 2

=

5× 1 + 1 × 4 5 × 3+ 1 × 2

6 × 1+ 3× 4 6 × 3+ 3× 2

=

9 17

18 24

Note that AB = BA.

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If A is an m × n matrix and B is an n × r matrix, then the product AB is the m × r matrix

whose entries are determined as follows.

To find the entry in row i and column j of AB, single out row i in matrix A and column j in

matrix B. Multiply the corresponding entries from the row and column and then add up the

resulting products.

Note: The product AB is defined only if the number of columns of A is the same as the number of rows of B.

Example 8

For A =

2 4

3 6

and B =

5

3

find AB.

Solution

A is a 2× 2 matrix and B is a 2 × 1 matrix. Therefore AB is defined.

The matrix AB is a 2 × 1 matrix.

AB =

2 4

3 6

5

3

=

2× 5+ 4× 3

3× 5+ 6× 3

=

22

33

Example 9

Matrix X shows the number of cars of models a and b bought by four dealers, A, B, C and D.

Matrix Y shows the cost in dollars of model a and model b.

Find XY and explain what it represents.

a b

X =

A

B

C

D

3 1

2 2

1 4

1 1

Y = 26 000

32 000 a

b

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Solution

a b

XY =

A

B

C

D

3 1

2 2

1 4

1 1

26 000

32 000

a

b

4 × 2 2× 1

The matrix XY is a 4× 1 matrix.

XY =

3 × 26 000+ 1 × 32 000

2 × 26 000+ 2× 32 000

1 × 26 000+ 4× 32 000

1 × 26 000+ 1× 32 000

=

110 000

116 000

154 000

58 000

The matrix XY shows that:dealer A spent $110 000

dealer B spent $116 000

dealer C spent $154 000

dealer D spent $58 000.

Example 10

For A = 2 3 4

5 6 7

and B = 4 0

1 20 3

find AB.

Solution

A is a 2 × 3 matrix and B is a 3 × 2 matrix. Therefore AB is a 2 × 2 matrix.

AB =

2 3 4

5 6 7

4 0

1 2

0 3

= 2

×4

+3

×1

+4

×0 2

×0

+3

×2

+4

×3

5× 4+ 6× 1 + 7 × 0 5 × 0+ 6× 2+ 7× 3

=

11 18

26 33

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Using the TI-Nspire

Multiplication of A =

3 6

6 7

and

B = 3 65 6.5

.

The products AB and BA are shown.

Using the Casio ClassPadMultiplication of

A =

3 6

6 7

and B =

3 6

5 6.5

.

AB and BA are shown.

Exercise 3C

1 If X =

2

−1

, Y =

1

3

, A =

1 −2

−1 3

, B =

3 2

1 1

, C =

2 1

1 1

, I =

1 0

0 1

,

Examples 8,10

find the products AX, BX, AY, IX, AC, CA, (AC)X, C(BX), AI, IB, AB, BA,

A2, B2

, A(CA) and A2C.

2 a Are the following products of matrices given in Question 1 defined?

AY, YA, XY, X2, CI, XI

b If A =

2 0

0 0

and B =

0 0

−3 2

, find AB.

3 Matrices A and B are 2× 2 matrices, and O is the zero 2 × 2 matrix. Is the following

argument correct?

‘If AB = O, and A = O, then B = O’.

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4 If L = [2 −1], X =

2

−3

, find LX and XL.

5 A and B are both m × n matrices. Are AB and BA defined and, if so, how many rows and

columns do they have?

6 Suppose

a b

c d

d −b

−c a

=

1 0

0 1

.

Show that ad − bc = 1. What is the product matrix if the order of multiplication on the

left-hand side is reversed?

7 Using the result of Question 6, write down a pair of matrices A, B such that

AB = BA = I, where I =

1 0

0 1

.

8 Select any three 2

×2 matrices A, B and C.

Calculate A(B+ C), AB+ AC and (B+ C)A.

9 It tak es John five minutes to drink a milkshake that costs $2.50, and 12 minutes to eat aExample 9

banana split that costs $3.00.

Calculate the product

5 12

2.50 3.00

1

2

and interpret the result in milk bar economics.

Suppose two friends join John.

Calculate

5 12

2.50 3.00

1 2 0

2 1 1

and interpret the result.

10 The reading habits of five students A, B, C , D and E are shown in the first matrix below

where the columns p, q, r , and s represent four weekly magazines. The second matrix

shows the cost in dollars of each magazine. Find the product of the two matrices and

interpret the result.

p q r s

A

B

C

D

E

0 0 1 1

1 0 1 1

1 0 0 0

1 1 1 1

0 1 0 1

p

q

r

s

2.00

3.00

2.50

3.50

11 Let S =

s11 s12 s13

s21 s22 s23

be the sales matrix for two showrooms selling three models of

cars. Here sij is the number of cars of model j sold from showroom i. Let the prices of the

three models of cars be $c1, $c2, $c3.

Call the 3 × 1 matrix, C =

c1

c2

c3

the price matrix.

a Find SC. b What is the practical meaning of SC?

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c Suppose the car dealer sells both new and used cars and the price of two-year-old used

cars for the three models is $u1, $u2 and $u3 respectively.

Form a new cost matrix.

C

=

c1 u1

c2 u2

c3 u3

Find SC and state its meaning.

d Suppose the car dealer makes 30% profit on his selling of new cars and 25% on used

cars.

If V =

0.3 0

0 0.25

, what is the meaning of CV?

3.4 Identities, inverses and determinants

for 2 × 2 matricesIdentitiesA matrix with the same number of rows and columns is called a square matrix. For square

matrices of a given dimension (e.g. 2 × 2) a multiplicative identity I exists.

For example, for 2× 2 matrices I =

1 0

0 1

and for 3× 3 matrices I = 1 0 0

0 1 00 0 1

If A =

2 3

1 4

, AI = IA = A, and this result holds for any square matrix multiplied by the

appropriate multiplicative identity.

InversesGiven a 2

×2 matrix A, is there a matrix B such that AB

=BA

=I?

Let B =

x y

u v

and A =

2 3

1 4

Then AB = I implies

2 3

1 4

x y

u v

=

1 0

0 1

i.e.

2 x + 3u 2 y + 3v

x + 4u y + 4v

=

1 0

0 1

∴ 2 x

+3u

=1 and 2 y

+3v

=0

x + 4u = 0 y + 4v = 1

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These simultaneous equations can be solved to find x, u, y, and v and hence B.

B =

0.8 −0.6

−0.2 0.4

B is said to be the inverse of A, as AB = BA = I.

Let A be a 2× 2 matrix with A =

a bc d

and let B =

x yu v

, where B is the inverse of A.

Then AB = I. In full this is written

ax + bu ay + bv

cx + du cy + dv

=

1 0

0 1

Hence ax + bu = 1 ay + bv = 0

cx + du = 0 cy + dv = 1

which form two pairs of simultaneous equations, for x, u and y, v respectively.

Taking the x , u pair and eliminating u , (ad − bc) x = d

Similarly, eliminating x , (bc−

ad )u =

c

These two equations can be solved for x and u respectively, provided ad − bc = 0.

x = d

ad − bcand u = c

cb − ad = −c

ad − bc

In a similar way it can be found that:

y = −b

ad − bcand v = −a

cb − ad = a

ad − bc

Therefore the inverse = d

ad

−bc

−b

ad

−bc

−cad − bc

aad − bc

.

The inverse of a square matrix A, is denoted by A−1. The inverse is unique.

ad − bc has a name, the determinant of A. This is denoted det(A).

For example,A =

a b

c d

, det(A) = ad − bc.

A 2× 2 matrix has an inverse only if det(A) = 0.

A square matrix is said to be regular if its inverse exists. Those square matrices which do

not have an inverse are called singular matrices; for a singular matrix det(A) = 0.

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Using the TI-NspireThe operation of matrix inverse is obtained by

raising the matrix to the power of −1.

The determinant command (b

>Matrix

and Vector>Determinant) is used as

shown.

(a is the matrix A =

3 6

6 7

defined

on page 73.)

Using the Casio ClassPadThe operation of matrix inverse is obtained

by entering A∧-1 in the entry line.

The determinant is obtained by typingdet( A).

Example 11

For the matrix A =

5 2

3 1

find:

a det(A) b A−1

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Solution

a det(A) = 5 × 1− 2× 3 = −1 b A−1 = 1

−1

1 −2

−3 5

=−1 2

3

−5

Example 12

For the matrix A =

3 2

1 6

find:

a det(A) b A−1 c X, if AX =

5 6

7 2

d Y, if YA =

5 6

7 2

Solution

a det(A)

=3

×6

−2

=16 b A−1

=1

16 6 −2

−1 3

c AX =

5 6

7 2

Multiply both sides (from the left)

by A−1.

A−1AX = A−1

5 6

7 2

∴ IX = X =1

16 6

−2

−1 35 6

7 2

= 1

16

16 30

16 0

=

1 2

1 0

d YA =

5 6

7 2

Multiply both sides (from the right) by

A−1.

YAA−1 = 1

16

5 6

7 2

6 −2

−1 3

∴

YI = Y =1

1624 8

40 −8

∴ Y =

3

2

1

25

2

−1

2

Exercise 3D

1 For the matrices A =

2 1

3 2

and B =

−2 −2

3 2

find:Example 11

a det(A) b A−1 c det(B) d B−1

2 Find the inverse of the following regular matrices ( is any real number, k is any non-zero

real number).

a

3 −1

4 −1

b

3 1

−2 4

c

1 0

0 k

d

cos −sin

sin cos

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3 If A, B are the regular matrices A =

2 1

0 −1

, B =

1 0

3 1

, find A−1

, B−1.

Also find AB and hence find, if possible, (AB)−1.

Also, from A−1, B−1, find the products A−1B−1 and B−1A−1. What do you notice?

4 Let matrix A = 4 32 1

.Example 12

a Find A−1. b If AX =

3 4

1 6

, find X. c If YA =

3 4

1 6

, find Y.

5 Let A =

3 2

1 6

, B =

4 −1

2 2

and C =

3 4

2 6

.

a Find X such that AX + B = C. b Find Y such that YA+ B = C.

6 If A is a 2

×2 matrix, a12

=a21

=0, a11

=0, a22

=0, then show that A is regular and

find A−1.

7 Let A be a regular 2 × 2 matrix, B a 2× 2 matrix and AB = 0. Show that B = 0.

8 Find all 2× 2 matrices such that A−1 = A.

3.5 Solution of simultaneous equationsusing matricesInverse matrices can be used to solve certain sets of simultaneous linear equations. Consider the equations

3 x − 2 y = 5

5 x − 3 y = 9

This can be written as3 −2

5 −3

x

y

=

5

9

If A =

3 −2

5 −3

the determinant of A is 3(−3)− 5(−2) = 1

which is not zero and so A−1 exists.

A−1 =−3 2

−5 3

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Multiplying the matrix equation

3 −2

5 −3

x

y

=

5

9

on the left hand side by A−1 and using

the fact that A−1A = I yields the following:

A−1 A x

y = A−1 5

9

∴ I

x

y

= A−1

5

9

∴

x

y

=

3

2

since A−1

5

9

=

3

2

This is the solution to the simultaneous equations.

Check by substituting x = 3, y = 2 in the equations.

When dealing with simultaneous linear equations in two variables which represent parallelstraight lines, a singular matrix results.

For example the system

x + 2 y = 3

−2 x − 4 y = 6

has associated matrix equation 1 2

−2

−4

x

y

=

3

6

Note that the determinant of

1 2

−2 −4

= 1 ×−4 − (−2 × 2) = 0.

There is no unique solution to the system of equations.

This situation will be considered in Chapter 8.

Example 13

If A

= 2 −1

1 2 and K

= −1

2, solve the system AX

=K where X

= x

y.

Solution

If AX = K , then X = A−1K

A−1K = 1

5

2 1

−1 2

×−1

2

=

0

1

∴ X =

0

1

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Example 14

Solve the following simultaneous equations.

3 x − 2 y = 6

7 x + 4 y = 7

Solution

The matrix equation is

3 −2

7 4

x

y

=

6

7

.

Let A =

3 −2

7 4

Then A−1 =1

26 4 2

−7 3

and

x

y

= 1

26

4 2

−7 3

6

7

= 1

26

38

−21

Using the TI-NspireEnter the matrices as shown.

Both the 2×

2 matrix template and

the 2 × 1 matrix template can be found

in the Math Templates, ( +on the Clickpad).

The matrix template can also be obtained

using +b>Math Templates

Note: It is also possible to use

solve

3 −2

7 4

x

y

=

6

7

, x

to find the values of x and y.

Using the Casio ClassPadEnter the expression shown in the entry line.

Note: If the matrices

3 −2

7 4

and

6

7

have already been entered as variables,

the variable letters may be used rather than

re-type the matrices.

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Exercise 3E

1 If A =

3 −1

4 −1

, solve the system AX = K , where X =

x

y

, and:Example 13

a K =−1

2

b K =

−2

3

2 If A =

3 1

−2 4

, solve the system AX = K , where:

a K =

0

1

b K =

2

0

3 Use matrices to solve the following pairs of simultaneous equations.Example 14

a −2 x + 4 y = 63 x + y = 1

b − x + 2 y = −1− x + 4 y = 2

c 2 x + 5 y = −10

y = x + 4

d 1.3 x + 2.7 y = −1.2

4.6 y − 3.5 x = 11.4

4 Use matrices to find the point of intersection of the lines given by the equations

2 x − 3 y = 7 and 3 x + y = 5.

5 Two children spend their pocket money buying books and CDs. One child spends $120 and

buys four books and four CDs. The other child buys three CDs and five books and spends$114. Set up a system of simultaneous equations and use matrices to find the cost of a

single book and a single CD.

6 Consider the system 2 x − 3 y = 3

4 x − 6 y = 6

a Write this system in matrix form as AX = K .

b Is A a regular matrix?

c Can any solutions be found for this system?

d How many pairs does the solution set contain?

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Chapter summary

A matrix is a rectangular array of numbers.

Two matrices A and B are equal when: each has the same number of rows and the same number of columns, and they have the same number or element at corresponding positions.

The size or dimension of a matrix is described by specifying the number of rows ( m) and the

number of columns (n). The dimension is written m × n.

Addition will be defined for two matrices only when they have the same dimension. The sum

is found by adding corresponding elements.a b

c d

+

e f

g h

=

a + e b + f

c + g d + h

Subtraction is defined in a similar way.

If A is an m × n matrix and k is a real number, k A is defined to be an m × n matrix whose

elements are k times the corresponding element of A.

k

a b

c d

=

ka kb

kc kd

If A is an m × n matrix and B is an n × r matrix, then the product AB is the m × r matrix

whose entries are determined as follows.

To find the entry in row i and column j of AB, single out row i in matrix A and column j in

matrix B. Multiply the corresponding entries from the row and column and then add up the

resulting products.

The product AB is defined only if the number of columns of A is the same as the number

of rows of B.

If A and B are square matrices of the same dimension and AB = BA = I then A is said to

the inverse of B and B is said to be the inverse of A.

If A =

a b

c d

then A−1 =

d

ad − bc

−b

ad − bc−c

ad

−bc

a

ad

−bc

det(A) = ad − bc is the determinant of matrix A.

A square matrix is said to be regular if its inverse exists. Those square matrices which do

not have an inverse are called singular matrices.

Simultaneous equations can be solved using inverse matrices, for example

ax + by = c

d x + ey = f

can be written asa b

d e x

y = c

f

and x

y = a b

d e−1 c

f

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Multiple-choice questions

1 The matrix A =

1 0

2 −1

−2 3

3 0

has dimension

A 8 B 4× 2 C 2 × 4 D 1× 4 E 3× 4

2 If A =

2 0

−1 3

and B =

1 −3 4

−1 −3 −1

then A + B =

A

3 −3

−2 0

B

3 4

−2 2

C

−1 2

2 3

D

2 1

1 −3

E cannot be determined

3 If C = 2

−3 1

1 0 −2

and D = 1

−3 1

2 3 −1

then D − C =

A

1 0 0

−1 −3 −1

B

2 −6 4

−2 0 −4

C

−1 0 0

1 3 1

D

1 −6 0

1 3 1

E cannot be determined

4 If M =−4 0

−2 −6

then −M =

A

−4 0

−2 −6

B

0 −4

−6 −2

C

4 0

−2 −6

D

0 4

6 2

E

4 0

2 6

5 If M =

0 2

−3 1

and N =

0 4

3 0

then 2M − 2N =

A

0 0

−9 2

B

0 −2

−6 1

C

0 −4

−12 2

D

0 4

12 −2

E

0 2

6 −1

6 If A and B are both m × n matrices, where m = n, then A + B is an

A m × n matrix B m ×m matrix C n × n matrix

D 2m × 2n matrix E cannot be determined

7 If P is an m × n matrix, and Q is a n × p matrix, the dimension of matrix QP is

A n × n B m × p C n × p D m × n E cannot be determined

8 The determinant of matrix A =

2 2

−1 1

is

A 4 B 0 C −4 D 1 E 2

9 The inverse of matrix A =

1 −1

1 −2

is

A −1 B

2 1−1 −1

C

1 1−1 −2

D

1 1−1 2

E

2 −11 −1

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10 If M =

0 −2

−3 1

and N =

0 2

3 1

then NM =

A 0 −4

−9 1 B

−4 −2

2 −8 C

0 4

9 1 D

−6 2

−3 −5 E

6 −2

−3 −5

Short-answer questions (technology-free)

1 If A =

0 2

3 4

and B =

1 3

0 5

, find:

a A+ B b A− B c AB d det(A) e A−1

2 If A= 1 0

2 3 and B

= −1 0

0 1, find:

a (A+ B)(A− B) b A2 − B2

3 Find all possible matrices A which satisfy the equation

3 4

6 8

A =

8

16

.

4 Let A =

1 2

3 −1

, B = [3 −1 2], C =

6

1

, D =

2 4

and E =

5

0

2

.

a State whether or not each of the following products exist: AB, AC, CD, BE

b Evaluate DA and A−1.

5 If A =

1 −2 1

−5 1 2

, B =

1 −4

1 −6

3 −8

and C =

1 2

3 4

, evaluate AB and C−1.

6 Find the 2× 2 matrix A such that A

1 2

3 4

=

5 6

12 14

.

7 If A

=

2 0 0

0 0 2

0 2 0

, find A2 and hence A−1.

8 If

1 2

4 x

is a singular matrix, find the value of x.

9 a If M =

2 −1

1 3

, find the value of:

i MM = M2 ii MMM = M3 iii M−1

b Find x and y, given that M x

y = 3

5.

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Extended-response questions

1 A =

3 1

1 −4

, B =

2 −1

5 2

a Find:i A+ B ii A− B iii 2A+ 3B iv C such that 3A + 2C = B

b Find:

i AB ii A−1 iii X such that AX = B iv Y such that YA = B

2 If A =

1 −2 2

2 0 −1

1 3 4

, B =

−2 0 1

4 2 −2

1 3 3

and C =

2 0 2

3 0 −1

1 3 1

, find:

a AB b AC c BC

d X such that AX

=C e Y such that YA

=B

f X such that AXC = CB g Y such that CYA = BA

3 a Consider the following system of equations:

2 x − 3 y = 3

4 x + y = 5

i Write this system in matrix form, as AX = K .

ii Find detA and A−1.

iii Solve the system of equations.

iv Interpret your solution geometrically.

b Consider the following system of equations:

2 x + y = 3

4 x + 2 y = 8

i Write this system in matrix form, as AX = K .

ii Find detA and explain why A−1 does not exist.

c Interpret your findings in part b geometrically.

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