Having difficulty with an Ax=b problem and understanding it

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Discussion Overview

The discussion revolves around solving matrix equations of the form Ax=b, particularly focusing on cases where the number of equations does not match the number of variables. Participants explore techniques for row reduction and the implications of having more equations than variables or vice versa.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about solving Ax=b when A has more rows than columns, specifically with a 3x2 matrix.
  • Another participant suggests constructing the augmented matrix and performing row reduction to find solutions.
  • A participant questions whether a provided matrix is in reduced row echelon form.
  • There is a query about solving for x when A is a 3x4 matrix and b is a 4x1 matrix, indicating a different structure of the problem.
  • Some participants note that having more unknowns than equations suggests the solutions may not be exact.
  • Another participant mentions the possibility of the matrix being inconsistent, leading to no solutions.
  • A participant shares their reduced matrix and seeks guidance on matching it to a given b vector.
  • One participant describes a method for obtaining least squares solutions when there are more or fewer equations than unknowns.

Areas of Agreement / Disagreement

Participants generally agree on the method of using row reduction to solve the matrix equations, but there are differing views on the implications of having more equations than variables and the nature of the solutions that arise from such cases. The discussion remains unresolved regarding the exact nature of solutions in these scenarios.

Contextual Notes

Participants express uncertainty about the setup of the problems, the interpretation of results, and the conditions under which solutions exist or do not exist. There are also unresolved mathematical steps related to matching reduced matrices with the vector b.

blhhblah
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I'm having trouble with Ax=b matrices. I get really confused when dealing with matrices that have a different number of rows from columns. An example is this problem:
(A is a 3x2 matrix)
A =
3 1
4 2
-5 -1

(b is a 3x1 matrix)
b =
1
0
-3
I need to find the solution set for x. Aren't there only 2 variables?
Thanks.
 
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Yes there are only two variables but 3 equations. Construct the augmented matrix A|b and perform row reduction until you get the reduced-row echelon form of the matrix. Now you can easily extract the solutions.
 
is this the reduced row echelon form?

1 0 1
0 1 -2
0 0 0
 
regarding another question...

how do you solve for x given A is a (3x4) matrix and b is a (4x1) matrix?
 
Yeah I got that as well. Now you can read the solutions for the variables directly from the matrix.
 
blhhblah said:
regarding another question...

how do you solve for x given A is a (3x4) matrix and b is a (4x1) matrix?
It's by using the same technique as well. But note that this time you have more unknowns than equations. What does that tell you about the nature of the solutions you will have?
 
i guess all the values won't be exact, but I'm not sure how to set this up
 
What do you mean by "exact"? Just write out the augmented matrix and again use row-reduction as before. Note that it is entirely possible that the matrix might be inconsistent, in that case there are no solutions.
 
matrix A was

2 8 4 16
1 1 2 2
1 -1 2 -2

I was able to reduce it to
1 0 2 0
0 1 0 2
0 0 0 0

but I'm not sure how to match it up to b, which is

-1
9
5
17
 
  • #10
Your first matrix was equaivalent to the set of equations
2x+ 8y+ 3z= 16, x+ y+ 2z= 2, and x- y+ 2z= -2 with b= <x, y, z>.

You have reduced it to a matrix equivalent to
x+ 2z= 0 and y= 2. What possible values of x, y, and z satisfy both of those?
 
  • #11
Whenever I have more or fewer equations than unknowns, I use the following to obtain the least squares answer. [itex]A^T A x = A^T b[/itex], where [itex]A^T[/itex] is the transpose of A.
 

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