Having difficulty with an Ax=b problem and understanding it

  • Context: Undergrad 
  • Thread starter Thread starter blhhblah
  • Start date Start date
  • Tags Tags
    Difficulty
Click For Summary
SUMMARY

The discussion focuses on solving linear equations represented by matrices in the form Ax=b, specifically dealing with matrices of different dimensions. The participants clarify that for a 3x2 matrix A and a 3x1 matrix b, one can construct the augmented matrix A|b and perform row reduction to find the solution set for the variables. They also address scenarios involving a 3x4 matrix A and a 4x1 matrix b, emphasizing the importance of understanding the nature of solutions when there are more variables than equations. The least squares solution method, ATA x = ATb, is recommended for cases with inconsistent systems.

PREREQUISITES
  • Understanding of matrix dimensions and their implications in linear equations
  • Familiarity with augmented matrices and row reduction techniques
  • Knowledge of reduced row echelon form (RREF)
  • Basic concepts of least squares solutions in linear algebra
NEXT STEPS
  • Study the process of constructing augmented matrices for various dimensions
  • Learn about row reduction techniques and how to achieve reduced row echelon form
  • Explore the least squares method for solving overdetermined systems
  • Investigate the implications of inconsistent systems in linear algebra
USEFUL FOR

Students and professionals in mathematics, engineering, and data science who are dealing with linear algebra problems, particularly those involving matrix equations and solution techniques.

blhhblah
Messages
6
Reaction score
0
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.
 
Physics news on Phys.org
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. A^T A x = A^T b, where A^T is the transpose of A.
 

Similar threads

  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 9 ·
Replies
9
Views
5K
  • · Replies 7 ·
Replies
7
Views
1K
  • · Replies 14 ·
Replies
14
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 24 ·
Replies
24
Views
2K