MATH 323 Lecture 2

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Systems of Equations as Matrices

Coefficient Matrix:

Augmented Matrix:

Elementary Row Operations

similar to Rules for Systems of Equations:

  1. Interchange two rows
  2. Multiply a row by a non-zero real number
  3. Replace a row by its sum with a multiple of another row

Solving Systems with Augmented Matrices: Gaussian Elimination

Convert to row echelon form (analogous to Strict Triangular Form):

Top row is called pivotal row since it is used to simplify the remaining rows
First non-zero entry in pivotal row is called pivotal element (should not be zero; better if 1)
Use pivotal row to make all elements in column below pivotal element zero
If at any point a pivot cannot be used for a certain column, skip it and continue with next column

Use 2nd row as pivotal row and −7 as pivotal element.

Simple Example

The first element in row 1 is 0, so it cannot be used as a pivotal row. Swap rows 1 and 2 since row 2 has a 1 in the first position.

From the resulting matrix, we can use back substitution on strict triangular form to solve for all unknowns.

Chapter 1.2: Row Echelon Form

A matrix is said to be in row echelon form if:

  1. The first non-zero entry in each non-zero row is 1
  2. If row does not consist entirely of zeroes, the number of leading zero entries in row is greater than the number of leading zero entries in row
  3. If there are rows whose entries are all zero, they are below the rows having non-zero entries

This matrix is not in strict triangular form, but the "staircase" made above all columns with zeroes and the fact that all pivot elements are 1 show that it is in row echelon form (similar to triangular form, but not strictly along diagonal.

This system is clearly inconsistent since the matrix states that and

However, changing two numbers (bottom-right; answers), makes the system consistent:

The columns that have pivotal elements (1, 3, and 5) correspond to lead unknowns (, , and ) Columns that do not have pivotal elements (2 and 4) correspond to free unknowns ( and )

The system has strict triangular form with respect to lead unknowns, and the system has infinite solutions bounded by following equations: (move all free unknowns to the right, leaving the lead unknowns on the left)

If we let and for , we can solve all unknowns in terms of two parameters:

Overdetermined Systems

Systems of equations are defined by

  • overdetermined (typically inconsistent)
  • underdetermined (typically infinite solutions)
  • → "normal" (typically one unique solution)

Reduced Row Echelon Form

An augmented matrix is said to be in reduced row echelon form if:

  1. it is in row echelon form
  2. the first non-zero entry in each row is the only non-zero entry in its column

How to achieve this? Consider the following system of equations and its augmented matrix:

The resulting row echelon form matrix is

Now we use the bottom row as the pivotal row and use it to eliminate all values above its pivotal element:

From this form, it is easy to find the solution set: