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Homework Help: Rank of matrix

  1. Oct 23, 2011 #1
    1. The problem statement, all variables and given/known data

    Given X E R^(n x k) and X has linearly independent columns, and A = X(X'X)^-1 X', I want to prove rank(A) = k. (A is the projection matrix basically).

    2. Relevant equations

    3. The attempt at a solution

    I was told to show im(A) = L(X).

    If i do that it would make sense since the Linear span of X is the basis of X with l.i columns, and dimL(X) = rank(X).

    How do i show images though. This has to be a product of a linear transformation in order for it to be an image doesn't it? I'm really unsure at that point.
  2. jcsd
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