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Section2.3Parametric Form

Objectives
  1. Learn to express the solution set of a system of linear equations in parametric form.
  2. Understand the three possibilities for the number of solutions of a system of linear equations.
  3. Recipe: parametric form.
  4. Vocabulary word: free variable.

Subsection2.3.1Free Variables

There is one possibility for the row reduced form of a matrix that we did not see in Section 2.2.

Example(A System with a Free Variable)

Consider the linear system

E 2 x + y + 12 z = 1 x + 2 y + 9 z = 1.

We solve it using row reduction:

C 2112 1129 1 D R 1 ←→ R 2 −−−−→ C 129 12112 1 D (Optional) R 2 = R 2 2 R 1 −−−−−−→ C 129 10 3 6 3 D (Step1c) R 2 = R 2 ÷− 3 −−−−−→ C 129 1012 1 D (Step2b) R 1 = R 1 2 R 2 −−−−−−→ C 105 1012 1 D (Step2c)

This row reduced matrix corresponds to the linear system

E x + 5 z = 1 y + 2 z = 1.

In what sense is the system solved? We rewrite as

E x = 1 5 zy = 1 2 z .

For any value of z , there is exactly one value of x and y that make the equations true. But we are free to choose any value of z .

We have found all solutions: it is the set of all values x , y , z , where

F x = 1 5 zy = 1 2 zz = zz anyrealnumber.

For instance, setting z = 0 gives the solution ( x , y , z )=( 1, 1,0 ) , and setting z = 1 gives the solution ( x , y , z )=( 4, 3,1 ) .

Figure2A picture of the solution set (the yellow line) of the linear system in this example. There is a unique solution for every value of z ; move the slider to change z .
Definition

Consider a consistent system of equations in the variables x 1 , x 2 ,..., x n . Let A be a row echelon form of the augmented matrix for this system.

We say that x i is a free variable if its corresponding column in A does not contain a pivot.

In the above example, the variable z was free because the reduced row echelon form matrix was

C 105 1012 1 D .

In the matrix

C 1 A 0 A A 001 A A D ,

the free variables are x 2 and x 4 . (The augmented column is not free because it does not correspond to a variable.)

Recipe: Parametric form

The parametric form of the solution set of a consistent system of linear equations is obtained as follows.

  1. Write the system as an augmented matrix.
  2. Row reduce to reduced row echelon form.
  3. Write the corresponding (solved) system of linear equations.
  4. Move all free variables to the right hand side of the equations.

Moving the free variables to the right hand side of the equations amounts to solving for the non-free variables (the ones that come from columns with pivots) in terms of the free variables. One can think of the free variables as being independent variables, and the non-free variables being dependent.

Implicit Versus Parameterized Equations

The solution set of the system of linear equations

E 2 x + y + 12 z = 1 x + 2 y + 9 z = 1

is a line in R 3 , as we saw in this example. These equations are called the implicit equations for the line: the line is defined implicitly as the simultaneous solutions to those two equations.

The parametric form

E x = 1 5 zy = 1 2 z .

can be written as follows:

( x , y , z )=( 1 5 z , 1 2 z , z ) z anyrealnumber.

This called a parameterized equation for the same line. It is an expression that produces all points of the line in terms of one parameter, z .

One should think of a system of equations as being an implicit equation for its solution set, and of the parametric form as being the parameterized equation for the same set. The parameteric form is much more explicit: it gives a concrete recipe for producing all solutions.

You can chose any value for the free variables in a (consistent) linear system.

Free variables come from the columns without pivots in a matrix in row echelon form.

Subsection2.3.2Number of Solutions

There are three possibilities for the reduced row echelon form of the augmented matrix of a linear system.

  1. The last column is a pivot column. In this case, the system is inconsistent. There are zero solutions, i.e., the solution set is empty. For example, the matrix
    A 10 001 000 1 B
    comes from a linear system with no solutions.
  2. Every column except the last column is a pivot column. In this case, the system has a unique solution. For example, the matrix
    A 100 a 010 b 001 c B
    tells us that the unique solution is ( x , y , z )=( a , b , c ) .
  3. The last column is not a pivot column, and some other column is not a pivot column either. In this case, the system has infinitely many solutions, corresponding to the infinitely many possible values of the free variable(s). For example, in the system corresponding to the matrix
    C 1 203 10014 1 D ,
    any values for x 2 and x 4 yield a solution to the system of equations.