1. Lesson 5.4
Solving System of Equations Using Matrices

2. In this lesson we will solve systems of equations using matrices.
This method is similar to the elimination method, but using matrices may be quicker because you keep track of equations using a shorter notation.
Numerals in the first equation match the numerals in the first row of the matrix.
Numerals in the second equation match the numerals in the second row of the matrix.

3. The solution to this system of equations is x = 4 and y = -7.
When we write the solution matrix we want it to represent the equations, therefore, x = 4 and y = -7 would look like this:
Numerals in the first equation match the numerals in the first row of the matrix.
Numerals in the second equation match the numerals in the second row of the matrix.

4. Row Operations in a Matrix
In the elimination method you combined equations and multiplied them by numbers.
In much the same way we can modify the rows of the matrix by performing row operations on each number in those rows.
Multiply (or divide) all numbers in a row by a non- zero number
Add all numbers in a row to corresponding numbers in another row
Add a multiple of the numbers in one row to the corresponding numbers in another row
Exchange two rows.

5. Ending matrix
Starting matrix
Solution can be read: x = 4 and y = -7

6. Solve the system of equations using matrices
Copy the numerals from each equation to form the matrix
Add -3 times row 1 to row 2

7. Divide row 2 by 7
Add 2 times row 2 to row 1
Therefore x = 7 and y = 2.

8. Diagonalization
In this investigation you will see how to combine row operations in your solution process.
Consider the system of equations:
Write the matrix for this system.
Describe how to use row operations to get 0 as the first entry in the second row.

9. Use row operations on the matrix from the last step to get 1 as the second number in row 2

10. Use row operations on the matrix from the last step to get 1 as the first number in row 1
What does the matrix mean? Give the solution to the system of equations.
x=4 and y = 3.

11. How do the first three rules for ROW OPERATIONS IN A MATRIX correspond to steps we used in the elimination process?
Multiply (or divide) all numbers in a row by a non-zero number
Add all numbers in a row to corresponding numbers in another row
Add a multiple of the numbers in one row to the corresponding numbers in another row
Exchange two rows.

12. Example
On Friday, 3247 people attended the county fair. The entrance fee for an adult was $5 and for a child 12 and under the fee was $3. The fair collected $14,273. How many of the total attendees were adults and how many were children.

13. Example A= number of adult attending C= number of children attending
Using total attendance: A+C=3247
Using money collected: 5A+3C=14273
Set up a matrix

14. Use row operations to simplify the matrix to the read the solution.
Add -5 times row 1 to row 2 to get a new row 2 (-5R1+R2)
Divide row 2 by -1 (R2/-2)
Add -1 times row 2 to row 1 to get a new row 1 (-1R1+R1)
There were 2266 adults and 981 children attending the fair on Friday.

15. Check that 2266 adults and 981 is a solution to both equations:
=3247
3247 = 3247
5(2266)+3(981)=14273
=14273
A+C=3247
5A+3C=14273

16. Solving a set of Matrices on the Graphing Calculator
Press 2nd Matrix and Edit Matrix A. First set the size and the entries.
Enter the matrix
Return to the home screen and press 2nd Matrix and choose Math B: rref(.
Complete the line to say rref([A]). Press Enter and you will see the solution matrix for this system.

17. In this Section You leaned to represent a linear system with a matrix.
You learned to use Row Operations to solve a system of linear equations.
You learned to solve a matrix on the graphing calculator.