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4x4 determinant using upper triangular form

  1. Feb 3, 2008 #1
    [SOLVED] 4x4 determinant using upper triangular form

    1. The problem statement, all variables and given/known data

    |2 0 1 4|
    |3 2 -4 -2|
    |2 3 -1 0|
    |11 8 -4 6|


    2. Relevant equations

    Ive done this a number of times and i still cant get the answer. I ended up w/ 275, but the answer is suppose to be zero

    3. The attempt at a solution

    1) 2 * |1 0 0.5 2|
    |3 2 -4 -2|
    |2 3 -1 0|
    |11 8 -4 6|

    2) 2* |1 0 0.5 2|
    |0 2 -5.5 -8|
    |0 3 -2 -4|
    |0 8 -9.5 -5|

    3) (2)(2)*|1 0 0.5 2|
    | 0 1 -2.75 -4|
    |0 3 -2 -4|
    |0 8 -9.5 -5|

    4) (2)(2)*|1 0 0.5 2|
    | 0 1 -2.75 -4|
    |0 0 6.25 8|
    |0 0 12.5 27|

    5) (2)(2)(6.25)* |1 0 0.5 2|
    |0 1 -2.75 -4|
    |0 0 1 (32/25)|
    |0 0 0 11|

    Determinant = 11*2*2*6.25 = 275
     
  2. jcsd
  3. Feb 3, 2008 #2

    Defennder

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    Homework Helper

    I would double-check your last row carefully for step 2.
     
  4. Feb 3, 2008 #3
    ah i see! Thanks for pointing that out.
     
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