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Homework Help: Least Squares

  1. Apr 16, 2007 #1
    This is a pretty basic question, but I just want to make sure. The question is to find the least square solution for
    [tex] \newcommand{\colv}[2] {\left(\begin{array}{c} #1 \\ #2 \end{array}\right)}
    1 & 2\\
    2 & 4
    \right) x = \colv{2}{2}[/tex]

    I can just find the orthogonal projection of the two vectors, right? In other words, use the find [tex]w = a_1\mathbf{v_1} + a_2\mathbf{v_2}[/tex]
    [tex]\newcommand{\colv}[2] {\left(\begin{array}{c} #1 \\ #2 \end{array}\right)}
    a_1 {\colv{1}{2}} + a_2 {\colv{2}{4}} = {\colv{w_1}{w_2}}[/tex]
    [tex] a_1 = \frac{<b,v_1>}{||v_1||^2}[/tex]
    [tex] a_2 = \frac{<b,v_2>}{||v_2||^2}[/tex]

    I don't really want to use the method with positive definite because the numbers turn out sticky.

    Then solution can be expressed as [tex]\mathbf{z} = \mathbf{b} - \mathbf{w}[/tex], such that z is orthogonal to the range of K?
    Last edited: Apr 16, 2007
  2. jcsd
  3. Apr 16, 2007 #2
    Two problems I just thought of though. If I want to do an orthogonal projection the basis vectors must be orthogonal. I suppose I could convert to an orthogonal basis.

    Furthermore, K^T K = Kx will still not give an answer such that b is in the range of K. No wonder this problem was assigned, it's not as basic as I thought. :(

    So if I convert the second vector to an orthogonal vector with Gram-Schmitt then I can use the orthogonal projection as the least squares answer?
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