Following vector in R3 are linear combination

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Homework Help Overview

The discussion revolves around determining whether a specific vector in R3 can be expressed as a linear combination of three given vectors. Participants are analyzing the implications of their calculations and the interpretation of results from an augmented matrix.

Discussion Character

  • Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants are examining the results of their row reduction on an augmented matrix and questioning the meaning of a row of zeros in relation to the existence of solutions. There is also a request for clarification on the method used to arrive at a particular solution.

Discussion Status

The discussion is ongoing, with participants providing differing interpretations of the results. Some guidance has been offered regarding the nature of solutions based on the row echelon form of the matrix, but no consensus has been reached on the calculations or conclusions drawn from them.

Contextual Notes

There is mention of an augmented matrix, but the specific setup and details of the calculations are not fully provided, leading to some ambiguity in the discussion.

hkus10
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Do you think the following vector in R3 are linear combination of

Code:
v1 = [4                                v2= [2                                v3 = [-2
      2                                     1                                      -1
     -3]                                   -2]                                      0]

Code:
[-2
 -1
  1]

The answer that I get is
Code:
[ 1 1/2 -1/2 -1/2
 -3  -2   0    1
  0   0   0   0   ]

Since I get a row of zeros, this means this is a inconsistent system which has no solution.

However, the book says that this vector has a combination of v_1, v_2, and v_3.

Did I make any error on my calculation?
 
Last edited:
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hkus10 said:
Do you think the following vector in R3 are linear combination of

Code:
v1 = [4                                v2= [2                                v3 = [-2
      2                                     1                                      -1
     -3]                                   -2]                                      0]

Code:
[-2
 -1
  1]

The answer that I get is
Code:
[ 1 1/2 -1/2 -1/2
 -3  -2   0    1
  0   0   0   0   ]

Since I get a row of zeros, this means this is a inconsistent system which has no solution.
No, that's not what it means at all. A row of zeros means that you have an infinite number of solutions. If you ended up with a row with a nonzero number on the right, then you would have an inconsistent system (you're working with an augmented matrix - you don't show it, but you are).

One solution I found was c1 = -1, c2 = 1, and c3 = 0.
hkus10 said:
However, the book says that this vector has a combination of v_1, v_2, and v_3.

Did I make any error on my calculation?
 


Mark44 said:
No, that's not what it means at all. A row of zeros means that you have an infinite number of solutions. If you ended up with a row with a nonzero number on the right, then you would have an inconsistent system (you're working with an augmented matrix - you don't show it, but you are).

One solution I found was c1 = -1, c2 = 1, and c3 = 0.


Can you show the way that your get the answer?
 


Here's your augmented matrix:
[tex]\begin{bmatrix} 4 & 2 & -2 & | & -2\\ 2 & 1 & -1 & | & -1 \\ -3 & -2 & 0 & | 1 \end{bmatrix}[/tex]

This matrix is shorthand for c1v1 + c2v2 + c3v3 = <-2, -1, 1>, where v1, v21, and v3 are as you had them in post #1.

The matrix you ended with in post 1 was correct as far as you went, but it wasn't in RREF (reduced row-echelon form), so continue reducing it as far as it will go. I ended up with two nonzero rows, the first of which was 1 0 -2 | -1. This row says that c1 - 2c3 = -1, or equivalently, c1 = 2c3 - 1. You should get a solution for c2 and c3 is a free variable.
 

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