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# Inverse of a Square Matrix

## 1. Learning Objectives for Section 4.5 Inverse of a Square Matrix

The student will be able to identify identity matrices formultiplication.

The student will be able to find the inverse of a square matrix.

The student will be able to work with applications of inverse

matrices such as cryptography.

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## 2. Identity Matrix for Multiplication

1 is called the multiplicative identity for real numbers sincea(1) = (1)a = a

For example 5(1) = 5

A matrix is called square if it has the same number of rows and

columns, that is, it has size n x n.

The set of all square matrices of size n x n also has a

multiplicative identity In, with the property

AIn = InA = A

In is called the n x n identity matrix.

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## 3. Identity Matrices

2 x 2 identity matrix:1 0

0 1

Barnett/Ziegler/Byleen Finite Mathematics 11e

3 x 3 identity matrix

1 0 0

0 1 0

0 0 1

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## 4. Identity Matrix Multiplication

AI = A (Verify the multiplication)We can also show that IA = A and in general AI = IA = A for

all square matrices A.

a11 a12 a13 1 0 0 a11 a12 a13

a

a

a

0

1

0

a

a

a

21

22

23

21

22

23

a a a 0 0 1 a a a

31 32 33

31 32 33

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## 5. Inverse of a Matrix

All real numbers (excluding 0) have an inverse.1

a 1

a

.

For example

1

5 1

5

Barnett/Ziegler/Byleen Finite Mathematics 11e

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## 6. Matrix Inverses

Some (not all) square matrices also have matrix inversesIf the inverse of a matrix A exists, we shall call it A-1

Then

1

1

A A A A In

Barnett/Ziegler/Byleen Finite Mathematics 11e

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## 7. Inverse of a 2x2 Matrix

There is a simple procedure to find the inverse of a two bytwo matrix. This procedure only works for the 2 x 2 case.

An example will be used to illustrate the procedure.

Example: Find the inverse of

2 3

1 2

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## 8. Inverse of a 2x2 matrix (continued)

Step 1: Determine whether or not the inverse actually exists.We define Δ = the difference of the product of the diagonal

elements of the matrix.

2 . 3

1 2

In order for the inverse of a 2 x 2 matrix to exist, Δ cannot

equal zero.

If Δ happens to be zero, then we conclude the inverse does

not exist, and we stop all calculations.

In our case Δ = 2(2)-1(3) = 1, so we can proceed.

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## 9. Inverse of a 2x2 matrix (continued)

2 31 2

2 3

1 2

Barnett/Ziegler/Byleen Finite Mathematics 11e

Step 2. Reverse the entries on

the main diagonal.

In this example, both entries

are 2, and no change is visible.

Step 3. Reverse the signs of

the other diagonal entries 3 and

1 so they become -3 and -1

Step 4. Divide each element

of the matrix by which in this

case is 1, so no apparent

change will be noticed.

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## 10. Inverse of a 2x2 matrix (continued)

The inverse of the matrix is then2 3

1 2

To verify that this is the inverse, we will multiply the original

matrix by its inverse and hopefully obtain the 2 x 2 identity

matrix:

2 3 2 3

1 2 1 2

Barnett/Ziegler/Byleen Finite Mathematics 11e

=

4 3 6 6 1 0

2 2 3 4 0 1

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## 11. Inverse of a General Square Matrix

1. Augment the matrix with the n x n identity matrix.2. Use elementary row operations to transform the matrix on the

left side of the vertical line to the n x n identity matrix. The

row operations are used for the entire row, so that the matrix

on the right hand side of the vertical line will also change.

3. When the matrix on the left is transformed to the n x n identity

matrix, the matrix on the right of the vertical line is the

inverse.

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## 12. Example: Inverse of a 3x3 Matrix

Find the inverse of1 1 3

2 1 2

2 2 1

Step 1. Multiply R1 by (-2) and add the result to R2.

Step 2. Multiply R1 by 2 and add the result to R3

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## 13. Example (continued)

Step 3. Multiply row 2 by (1/3) to get a 1 in the secondrow, first position.

Step 4. Add R2 to R1.

Step 5. Multiply R2 by 4 and add the result to R3.

Step 6. Multiply R3 by 3/5 to get a 1 in the third row, third

position.

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## 14. Example (continued)

Step 7. Eliminate the (5/3) in the first row, third position bymultiplying R3 by (-5/3) and adding result to R1.

Step 8. Eliminate the (-4/3) in the second row, third position

by multiplying R3 by (4/3) and adding result to R2.

Step 9. You now have the identity matrix on the left, which

is our goal.

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## 15. Example Solution

The inverse matrix appears on the right hand side of thevertical line and is displayed below. Many calculators as well

as computers have software programs that can calculate the

inverse of a matrix quite easily. If you have access to a TI 83,

consult the manual to determine how to find the inverse using

a calculator.

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## 16. Application: Cryptography

Matrix inverses can provide a simple and effective procedure forencoding and decoding messages. To begin, assign the numbers

1-26 to the letters in the alphabet, as shown below. Also assign

the number 0 to a blank to provide for space between words.

Blank A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Thus the message “SECRET CODE” corresponds to the sequence

19 5 3 18 5 20 0 3 15 4 5

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## 17. Cryptography (continued)

Any matrix A whose elements are positive integers andwhose inverse exists can be used as an encoding matrix.

For example, to use the 2 x 2 matrix

4 3

A

1

1

to encode the message above, first divide the numbers in

the sequence 19 5 3 18 5 20 0 3 15 4 5 into groups

of 2, and use these groups as the columns of a matrix B:

19 3 5 0 15 5

B

5

18

20

3

4

0

We added an extra blank at the end of the message to

make the columns come out even.

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## 18. Cryptography (continued)

Then we multiply this matrix on the left by A:4 3 19 3 5 0 15 5

AB

5 18 20 3 4 0

1

1

91 66 80 9 72 20

24

21

25

3

19

5

The coded message is

91 24 66 21 80 25 9 3 72 19 20 5

This message can be decoded simply by putting it back into matrix

form and multiplying on the left by the decoding matrix A-1.

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