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Math Help - Lu factorization opper matrix form problem

  1. #1
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    May 2009
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    Lu factorization opper matrix form problem

    basically i have to solve

    4 -1 2 x1 = 15
    -1 2 3 x2 = 5
    5 -7 9 x3 =8

    i know all the steps from what i gt from my lectures. can anyone of you help me just how to write in the A=LU form

    this is what i have gt from my lectures

    A = L U

    4 -1 2 4 0 0 1 -(1/4) (1/2)
    -1 2 3 -1 L(22) 0 0 1 U(23)
    5 -7 9 5 L (32) L(33) 0 0 1



    can you just show me how he gt the -1/4 and 1/2 in U .
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  2. #2
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    May 2009
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    Ok took me a while to read your matrices but turns out the answer to your question is quite simple. The product of the L and U matrices must return you original matrix A. So the two terms you are asking about are arrived at since:

    L(1,1) = A(1,1) = 4

    So then

    U(1,2) = \frac{A(1,2)}{L(1,1)} = \frac{-1}{4}
    U(1,3) = \frac{A(1,3)}{L(1,1)} = \frac{2}{4} = \frac{1}{2} .

    If you need any more help on LU factorisation put up another post.
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