What is a linear equation ?

What does it mean to solve a system of linear equations?

What are the possible cases when solving a system of linear equations ?

**Example 1:** Solve the following system of linear equations:

4x-5y = -30

2x+y = -8

**Example 2:** Solve the following system of linear equations:

2x-y = 1

6x-3y = 12

**Example 3:** Solve the following system of linear equations:

3x-7y = 4

6x-14y = 8

**Note: We will first learn how to set-up the system in these word problems.
We will later return to**

learn how to systematically solve these problems.

**Example 4:** Jennifer has made it through her ten year class reunion.
She is wanting to remember

how many former classmates, spouses, and former teachers attended the reunion.
She has

lost her records but recalls that the ticket sales totaled $4,775. She charged
$30 for each former

classmate, $25 for each spouse, and $20 for each former teacher. She recalls
that the number

of former classmates and spouses combined was 130 more than the number of former
teachers.

She also recalls that there were five times as many former classmates there as
spouses.

Help Jennifer remember the number of former classmates, spouses, and former
teachers that

attended the reunion.

Set-up the linear system:

**Example 5: **An investment club has $200,000 earmarked for investment in
stocks. To arrive at

an acceptable overall level of risk, the stocks that management is considering
have been classified

into three categories: high-risk, medium-risk, and low-risk. Management
estimates that

high-risk stocks will have a rate of return of 15%/year; medium-risk stocks,
10%/year; and

low-risk stocks, 6%/year. The members have decided that the investment in
low-risk stocks

should be equal to the sum of the investments in the stocks of the other two
categories . Determine

how much the club should invest in each type of stock if the investment goal is
to have

a return of $20,000/year on the total investment. (Assume that all of the money
available for

investment is invested.)

Set-up the linear system:

**Definitions:**

•A **matrix** is an ordered rectangular array of numbers. A matrix with m rows
and n columns has size

m*n. The entry in the ith row and jth column is denoted by a_{ij}.

**•Augmented Matrix:** We can combine two matrices into one, visually
separating them by a vertical

line. This is useful when solving a system of equations.

**Gauss-Jordan Elimination**

This method allows us to strategically solve systems of linear equations. We
perform operations

on an augmented matrix that is formed by combining the coefficient matrix and
the

constant matrix as shown in the next example.

**Example 6:** Find the intial augmented matrix for the system of
equations below:

a)

2x-4y = 10

y = 1-3x

The goal of the Gauss-Jordan Elimination Method is to get the augmented
matrix into Row

Reduced Echelon Form. A matrix is in Row Reduced Echelon Form when:

1. Each row of the coefficient matrix consisting entirely of zeros lies below
any other row having

nonzero entries.

2. The first nonzero entry in each row is 1 (called a leading 1)

3. In any two successive (nonzero) rows, the leading 1 in the lower row lies
to the right of the leading

1 in the upper row.

4. If a column contains a leading 1, then the other entries in that column
are zeros.

**Note:** We only consider the coefficient side (left side) of the
augmented matrix when determining

whether the matrix is in row-reduced echelon form.

**Example 7:** Are the following in Row Reduced Echelon form?

To put a matrix in Row Reduced Echelon Form, there are three valid Row
Operations :

1. Interchange any two rows

2. Replace any row by a nonzero constant multiple of itself (cR_{i})

3. Replace any row by the sum of that row and a constant multiple of any other
row

Steps for Gauss Jordan Elimination:

1. Begin by transforming the top left corner element, a_{11}, into 1. This is your
first pivot element.

2. Next, transform the other elements in its column into zeros using the 3 row
operations.

3. Choose the next pivot element (diagonal down from the first pivot element)

4. Turn this 2nd pivot element into a 1, and transform the rest of its column
into zeros.

5. Continue until the coefficient matrix resembles the identity matrix(1’s along
the main-diagonal and

0’s everywhere else.)

**Example 8: **Solve the following system of equations
using Gauss Jordan Elimination:

3x+y = 1

-7x-2y = -1

**Example 9:** Solve the system that we set-up in
Example 5.

**Example 10:** Solve the system that we set-up in
Example 6.

**Note:** Our calculator will put an augmented matrix
into row-reduced form for us. This only

works when the # of rows is less than or equal to # of columns. Below are the
steps you must

follow:

1. Enter the augmented matrix into your calculator.

2. Go to your home screen.

3. Press MATRIX , cursor right to MATH , and select B:rref.

4. Call the matrix you want to reduce and hit ENTER.

We will use the calculator for working the problems in the
next section. You are responsible

for knowing how to do the Gauss-Jordan Method by hand, but always remember that
you can

check your work with the calculator.

Section 4.3 Homeowork Problems: 1, 5, 9, 13, 17, 21, 25,
27, 29, 31, 35, 37, 39, 41, 45

We are using the same process from Section 4.3 but now our
solutions will not always be unique.

**Example 1:** Solve the following system:

x+y+-2z = -3

2x-y+3z = 7

x-2y+5z = 0

**Example 2: **Solve the following system:

x+2y-3z = -2

3x-y-2z = 1

2x+3y-5z = -3

**Example 3:** Solve the following systems:

a)

x+y = 7

2x+3y = 8

4x-y = 3

b)

x+y = 7

2x+3y = 8

-5x-5y = -35

**Theorem:** We have the following theorem concerning
the number of equations and the number of

variables in a system :

•If the number of equations is greater than or equal to
the number of variables, the system will have

either a unique solution, no solution, or infinitely many solutions.

•If the number of equations is fewer than the number of variables, the system
will have either no

solution or infinitely many solutions.

**Example 4:** Solve the following system:

Section 4.4 Homework Problems: 7, 9, 11, 13, 15, 17, 21,
25, 29, 33, 37, 41, 45, 51, 53, 57