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Homework Help: Elementary row operations question

  1. Sep 1, 2005 #1
    I am working with the following equation:
    |1 -2 3|
    |6 -11 15|
    |4 -14 21|
    a. add -4 time Row1 to Row 3
    b. add -6 times Row 1 to Row 2
    c. add 6 times Row 2 to Row 3
    d. multiply row 3 by -1/9

    I have come up with the following answer:
    |24 -48 72|
    |180 -354 522|
    |-19.99 39.99 -58.99|

    Where did I go wrong?
     
  2. jcsd
  3. Sep 1, 2005 #2
    Incidently, this is a multiple choice question and my answer isn't anywhere near any of the answers to choose from.
     
  4. Sep 1, 2005 #3

    Fermat

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    After the 1st 2 operations, you should have ended up with,

    Code (Text):
    1 -2 3
    0 1 -3
    0 -6 9
     
  5. Sep 1, 2005 #4
    When you say first two operations, do you mean a and b or a?

    What am I messing up?

    After I ran a, I had:

    -4 8 -12
    6 -11 15
    0 -6 9
     
  6. Sep 1, 2005 #5

    VietDao29

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    Why are you multiplying the first row by -4???
    It says add -4 times Row 1 to Row 3.
    Viet Dao,
     
  7. Sep 1, 2005 #6
    What does that mean?
    Do I add -4 to Row 3 or do I multiply -4 by Row 1 than add Row 1 to Row 3?
     
  8. Sep 1, 2005 #7

    VietDao29

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    Add -4 times Row 1 To Row 3 means that you change Row 3 by adding -4 times Row 1 to it, and you don't change anything in Row 1, (ie: you just change Row 3).
    Row 1 is: |1 -2 3|
    -4 times Row 1 is : |-4 8 -12| (*)
    You will add the (*) to row 3, which is:
    |4 + (-4)xxxxxx-14 + 8xxxxxx21 + (-21)| = |0 -6 9|
    You still keep Row 1. You only change Row 3. ie:
    |1 -2 3|
    |6 -11 15|
    |0 -6 9|
    Do the same for b, c, and d.
    Viet Dao,
     
  9. Sep 1, 2005 #8
    OH, I SEE!

    I have come up with:
    1 -2 3
    0 1 -3
    0 0 1

    THANK YOU!
     
  10. Sep 1, 2005 #9
    :rofl: The light bulb has come on
     
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