Find the complete solution to Ax = b, b = (-1,0,1)

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s3a
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Homework Statement


Find the complete solution to Ax = b for b = (-1,0,1).

Homework Equations


Reduced-row echelon form procedure.
Matrix multiplication procedure.

The Attempt at a Solution


I have no idea what is being done in the solution attached (other than the reduced-row echelon and matrix multiplication procedures).

If someone could help me understand what the attached solution is saying, I would very much appreciate it!
 

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Answers and Replies

  • #2
HallsofIvy
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The row reduction gives
[tex]\begin{bmatrix}1 & 2 & 3 & 4 & b_1 \\ 0 & 1 & 2 & 3 & \frac{3}{2}b_1-\frac{1}{2}b_2 \\ 0 & 0 & 0 & 0 & -\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3 \end{bmatrix}[/tex].

That last line is equivalent to
[tex]0b_1+ 0b_2+ 0b_3= -\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3[/tex]
The left side is clearly 0 for any [itex]b_1[/itex], [itex]b_2[/itex], [itex]b_3[/itex] so, in order for this equation to be true, the right side must also be 0: [itex]-\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3= 0[/itex]. Since we could choose any values for two of [itex]b_1[/itex], [itex]b_2[/itex], [itex]b_3[/itex] and solve that equation for the other, the solution set had dimension 2.
 
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The row reduction gives
[tex]\begin{bmatrix}1 & 2 & 3 & 4 & b_1 \\ 0 & 1 & 2 & 3 & \frac{3}{2}b_1-\frac{1}{2}b_2 \\ 0 & 0 & 0 & 0 & -\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3 \end{bmatrix}[/tex].

That last line is equivalent to
[tex]0b_1+ 0b_2+ 0b_3= -\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3[/tex]
The left side shouldn't be in terms of b1, etc.

The last line in the augmented matrix above represents ##0x_1 + 0x_2 + 0x_3 + 0x_4 = -\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3##
As stated below, the left side is identically zero for any choices of ##x_1, x_2, x_3, x_4##, which means that the right side must be zero. Since we have one equation (##-\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3 = 0##) in three variables, two of the variables are arbitrary, which gives us a solution space of dimension 2.
HallsofIvy said:
The left side is clearly 0 for any [itex]b_1[/itex], [itex]b_2[/itex], [itex]b_3[/itex] so, in order for this equation to be true, the right side must also be 0: [itex]-\frac{1}{4}b_1+ \frac{1}{2}b_2- \frac{1}{4}b_3= 0[/itex]. Since we could choose any values for two of [itex]b_1[/itex], [itex]b_2[/itex], [itex]b_3[/itex] and solve that equation for the other, the solution set had dimension 2.
 

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