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Homework Help: Finding a basis and dimension of a subspace

  1. May 3, 2010 #1
    1. The problem statement, all variables and given/known data

    Let S={v1=[1,0,0,0],v2=[4,0,0,0],v3=[0,1,0,0],v4=[2,-1,0,0],v5=[0,0,1,0]}

    Let W=spanS. Find a basis for W. What is dim(W)?

    2. Relevant equations

    3. The attempt at a solution

    i know that a basis is composed of linearly independent sets. This particular problem's basis cannot be greater than 4 since the last row of all of the column vectors is 0. i have looked all throughout my book and online through this site and i cannot figure out how to even start this problem. Please help me :(
  2. jcsd
  3. May 3, 2010 #2
    You want to find a set that spans W and consists of linearly independent vectors. Are the vectors {v1, ..., v5} linearly independent? Use the method of setting a1v1 + .... + a5, v5 = 0, where the ai's are scalars, to check this.

    A basis is a set composed of linearly independent vectors (and it must also span the space)
    Last edited: May 3, 2010
  4. May 3, 2010 #3
    no, they arent because the variable c3 and c4 depend on each other in row 2 of the matrix after setting c1v1+c2v2+c3v3+c4v4+c5v5=0.
  5. May 3, 2010 #4
  6. May 3, 2010 #5
    When I do this, a1v1 + .... + a5, v5 = 0, i end up getting -a1=4a4; a3=a4=a5=0;
  7. May 4, 2010 #6


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    The set of all [a, b, c, d] is four dimensional so 5 vectors can't be independent. It should be easy to see that v2= 4v1 so you can just drop v2. v5 is independent of all the others because it has a "1" in the third place while all others have "0".

    Finally, v4= [2, -1, 0, 0]= 2[1, 0, 0, 0]- 1[0, 1, 0, 0]= 2v1- v3 so you can drop v4. v1 and v3 are obviously independent.
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