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Homework Help: Subspace question

  1. Mar 18, 2013 #1
    1. The problem statement, all variables and given/known data

    Determine whether the following is a Subspace of r^3:

    All vectors of the form (a,b,c), where b=a+c

    3. The attempt at a solution

    The answer in the book says it is not a subspace but I can only find examples that show it is a Subspace I.e.

    Let u=(a,a+c,c)=(1,2,1), v=(a,a+c,c)=(2,4,2) and k=2 then

    U+v=(3,6,3)=(a,a+c,c) so it's closed under addition

    Ku=2(a,a+c,c)=2(1,2,1)=(2,4,2)=(a,a+c,c) so it is closed under scalar multiplication

    Maybe I don't understand the concept correctly am I doing something wrong?
  2. jcsd
  3. Mar 18, 2013 #2


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    Science Advisor

    If your textbook says this is not a subspace, it is wrong. It is, exactly as you say, the subspace of R3 of all vectors of the form (a, b, c)= (a, a+ c, c)= (a, a, 0)+ (0, c, c)= a(1, 1, 0)+ c(0, 1, 1). In other words, it is the two dimensional subspace with (1, 1, 0) and (0, 1, 1) as basis.
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