Proving Linear System Ax=b Consistent iff Rank A = m

In summary, a matrix equation of the form Ax=b is consistent for all column vectors b if and only if the rank of A is equal to the number of rows in the matrix. This means that the equation has at least one solution for any vector b, and that b must be in the column space of A.
  • #1
eyehategod
82
0
let A be a mxn matrix.
prove that the system of linear equations Ax=b is consistnet for all column vectors b if and only if the rank of A is m.

I have no idea how to start, can anyone helo me out?
 
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  • #2
what does it mean if the matrix equation is consistent for all vectors b?
 
  • #3
i guess my problem is that i don't quite understand when it says "consistent for all column vectors b."
 
  • #4
also it would mean that b is in the column space of A.
 
  • #5
eyehategod said:
also it would mean that b is in the column space of A.

yes but any b?
 
Last edited:
  • #6
eyehategod said:
i guess my problem is that i don't quite understand when it says "consistent for all column vectors b."

A system of linear equations is consistent if it has a solution. Of course, this solution need not be unique.
 
  • #7
radou said:
A system of linear equations is consistent if it has a solution. Of course, this solution need not be unique.
A matrix equation, Ax= b, is "consistent" if it has at least one solution. "Ax= b is consistent for all b" means the equation Ax= b is consistent no matter what vector b is.

The OP said earlier, "also it would mean that b is in the column space of A." Okay. And if b is to be any member of A, what must the column space be?
 

What does it mean for a linear system to be consistent?

A linear system is consistent if it has at least one solution. This means that there is a set of values for the variables that satisfies all of the equations in the system.

What is the rank of a matrix?

The rank of a matrix is the number of linearly independent rows or columns in the matrix. In other words, it is the number of equations or variables in the system that are not redundant or dependent on each other.

How do you prove that a linear system Ax=b is consistent?

To prove that a linear system Ax=b is consistent, you need to show that the rank of the coefficient matrix A is equal to the number of variables in the system. This can be done through various methods such as row reduction or the use of the Rank-Nullity Theorem.

What does it mean for a linear system to be inconsistent?

A linear system is inconsistent if it has no solution. This means that there is no set of values for the variables that satisfies all of the equations in the system. In other words, the equations in the system are contradictory and cannot be satisfied simultaneously.

Why is proving the consistency of a linear system important?

Proving the consistency of a linear system is important because it allows us to determine whether the system has solutions or not. It also helps us to understand the relationships between the variables and equations in the system and can lead to further insights and applications in various fields of science and engineering.

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