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Homework Help: Formulating an LP problem (Mathematics/Engineering)

  1. Sep 28, 2010 #1
    1. The problem statement, all variables and given/known data

    I don't exactly have a specific example, I'm looking for the general idea behind LP constraints.

    2. Relevant equations

    Within the constraints, do the variables have to be all to one side?

    For instance, X1 + 2 > 3 is a standard constraint

    would for example

    X1 + 2 > X1 + 1

    still be acceptable as a constraint?
     
  2. jcsd
  3. Sep 28, 2010 #2

    Mark44

    Staff: Mentor

    This would normally be written as x1 > 1. Did you mean x1 + 2x2 > 3?
    No. This is equivalent to 2 > 1, which is always true, so doesn't constrain things in any way. Did you mean x1 + 2 > x2 + 1?

    That could be written as x1 - x2 > -1
     
  4. Sep 28, 2010 #3
    I'm sorry, it was late and my examples are horrible. They do not confine to standard constraints, they were simply just numbers to illustrate.

    The basic question is, can you have variables on both sides of a constraint? Or must the variables be confined to one side?
     
  5. Sep 28, 2010 #4

    Mark44

    Staff: Mentor

    You can always move variables from one side of an equation/inequality to the other.
     
  6. Sep 28, 2010 #5
    Yes I understand that, my question is, must I?

    I recently took a test, and after looking through the examples, they set the constraints equal to 0 and isolated all the variables to one side. I left the constraints with variables on both sides. Not sure if I would get points off for doing so...
     
  7. Sep 28, 2010 #6

    Mark44

    Staff: Mentor

    To use matrix methods (e.g., Simplex tableau), you need to have all the variables on one side, and the constant on the other. I'm pretty sure this is correct, although I haven't done one of these problems for a good long while.
     
  8. Sep 28, 2010 #7
    Gotcha yeah, that makes sense. I suppose it wouldn't be completely incorrect, yet not properly done. Anyway I'll find out soon enough.

    Thanks for your help!
     
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