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Given a homogeneous system, what can we say about a similar but nonhomogeneous system?

  1. Nov 16, 2014 #1

    kosovo dave

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    This might belong in the HW section, but since it's specific to Linear Algebra I posted it here.

    Alright, so we have a homogeneous system of 8 equations in 10 variables (an 8 x 10 matrix, let's call it A). We have found two solutions that are not multiples of each other (lets call them a and b), and every other solution is a linear combination of them. Can you be certain that any nonhomogeneous equation with the same coefficients has a solution?

    I want to say yes, but I'm not sure why. Here's the stuff I know:
    - Our solution for the homogeneous system is span{a, b}.
    - Since there are free variables/the null space is not just 0 we know there are nontrivial solutions.
    - Dim(Null(A))=8
     
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  3. Nov 16, 2014 #2

    Simon Bridge

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    Try constructing a non-homogeneous equation with those two coefficients that does not have a solution - consider that the inhomogeniety can be anything.
     
  4. Nov 16, 2014 #3

    kosovo dave

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    I don't know why, but I'm still having a hard time with this :/ Could you give me another hint?

    Here's what I tried:
    Ax=c

    if c= (0,0,1,0,0,0,0,0,0,0) I think the system would be inconsistent because row 3 of the augmented matrix would be all 0's and then a nonzero to the right of the vertical line. Does that work?
     
  5. Nov 16, 2014 #4

    Simon Bridge

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    Well, how would you normally find the solution to a non-homogeneous system knowing the solution to the homogeneous one?
     
  6. Nov 16, 2014 #5

    kosovo dave

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    augment the nonhomogeneous system with a solution from the homogeneous one?
     
  7. Nov 22, 2014 #6

    Stephen Tashi

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    For a matrix [itex] A [/itex] and a column vector [itex] x [/itex] the result of [itex] Ax [/itex] can be viewed as a linear combination of the column vectors of [itex] A [/itex] where the coefficients in the linear combination are the entries of [itex] x [/itex]. So if [itex] Ax = b [/itex] has a solution, the vector [itex] b [/itex] must be in the span of the column vectors.
     
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