Following vector in R3 are linear combination

hkus10
Messages
50
Reaction score
0
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:
Physics news on Phys.org


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:
\begin{bmatrix} 4 & 2 & -2 & | & -2\\ 2 & 1 & -1 & | & -1 \\ -3 & -2 & 0 & | 1 \end{bmatrix}

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.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
Back
Top