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Linear Algebra - Rank Theorem

  • Thread starter sami23
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  • #1
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Homework Statement


Suppose a non-homogeneous system, Ax = b, of 3 linear equations in 5 unknowns (3x5 matrix) and 3 free variables, prove there is no solution for any vector b.


Homework Equations


Using the rank theroem:
n = rank A + dim Nul(A) where n = # of columns; dim Nul(A) = # free variables


The Attempt at a Solution


rank A = n - dim Nul(A) = 5-3 = 2 (which represents the pivot columns)

How do I know there are no solutions for any vector b knowing there can be at most 3 pivot columns?
 

Answers and Replies

  • #2
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This doesn't make sense. Most systems of 3 equations in 5 unknowns have many solutions.
 
  • #3
Ray Vickson
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Homework Statement


Suppose a non-homogeneous system, Ax = b, of 3 linear equations in 5 unknowns (3x5 matrix) and 3 free variables, prove there is no solution for any vector b.


Homework Equations


Using the rank theroem:
n = rank A + dim Nul(A) where n = # of columns; dim Nul(A) = # free variables


The Attempt at a Solution


rank A = n - dim Nul(A) = 5-3 = 2 (which represents the pivot columns)

How do I know there are no solutions for any vector b knowing there can be at most 3 pivot columns?

If the matrix has rank 3 there are infinitely many solutions.

RGV
 

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