1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gauss-jordan 3 equation matrix help

  1. Sep 14, 2012 #1
    1. The problem statement, all variables and given/known data

    the original problem is in matrix form:

    1 1 2 8
    -1 -2 -3 1
    3 -7 4 10

    2. Relevant equations



    3. The attempt at a solution

    i got all the way to :

    1 1 0 -4
    0 1 0 1
    0 0 1 2

    however, the top row is supposed to = 3 but when i subtract row 2 from row 1 i get -5....i can't find where i messed up in the steps i did it. anyone get the same thing as me or know what i did wrong?
     
  2. jcsd
  3. Sep 14, 2012 #2

    Mark44

    Staff: Mentor

    I don't know what you did to get the matrix above, but it's incorrect. This says that y = 1 and z = 2. From these you can work back to get x.

    In any case, these values are incorrect. I've worked through the row reduction, and have a solution, which I have checked, so I know it's right.

    Can you show what work you did to get the above? From that, we can tell you where you went astray.
     
  4. Sep 15, 2012 #3
    that's weird because the book answer says that x = 3, y = 1, and z = 2, which is why i thought i only got the top row part wrong. the operations i used were:

    row 2 * -1 minus row 1

    row 3 minus 3 * row 1

    10 * row 2 + row 3

    row 3 / -52

    5 * row 3 plus row 2

    2 * row 3 minus row 1

    row 2 minus row 1
     
  5. Sep 15, 2012 #4

    Mark44

    Staff: Mentor

    That is not a solution to the problem you posted.
    The augmented matrix you posted is
    1 1 2 8
    -1 -2 -3 1
    3 -7 4 10

    The book solution works in the 1st and 3rd equations, but it doesn't work in the 2nd equation.

    -1(3) -2(1) -3(2) = -3 -2 -6 = -11 ≠ 1

    There are a couple of possibilities. It might be that you mistyped the original matrix. If the 2nd row was actually -1 -2 -3 11, then the book's answer is correct.


    Otherwise, the book's answer is wrong.
     
  6. Sep 15, 2012 #5

    You are correct, i accidentally put a -3 in the second row when it should have been a 3. My apologies to everyone. Also, is it a valid row operation to divide a row by itself?
     
    Last edited: Sep 15, 2012
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Gauss-jordan 3 equation matrix help
Loading...