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ptc-53413201-Visitor
1-Visitor
October 25, 2013
Solved

This problem has 12 variables and I do have 12 equations, but I still get the "variable undefined" message

  • October 25, 2013
  • 15 replies
  • 5870 views

Hey folks, I am posting this because I am really having trouble seeing where the problem is within this worksheet. Any help would be greatly appreciated.

Best answer by Werner_E

See attached.

BTW, there is nothing wrong with using the Align Regions facilitiy in Mathcad!

15 replies

ValeryOchkov24-Ruby IV
24-Ruby IV
October 25, 2013

a2.png

Werner_E
Werner_E25-Diamond IAnswer
25-Diamond I
October 25, 2013

See attached.

BTW, there is nothing wrong with using the Align Regions facilitiy in Mathcad!

    ptc-53413201-VisitorAuthor
    1-Visitor
    October 26, 2013

    So my problem was that I needed to have the mass fractions as x.2a instead of a.2 (and so on for the others) in the find block?

    Werner_E
    Werner_E25-Diamond I
    25-Diamond I
    October 26, 2013

    You had used those fractions x.2a etc. in the equations of your solve block. Those variables were not used as solve for variables and they had no value, were undefined. So Mathcad could not evaluate those equations in the solve provess and throws the error.

    There are two other ways to make it work:

    1) change x.2a etc. to a.2 in the equations of the solve block

    2) assign guess values to x.2a, too, and solve for all 19 variables. Not recommended but should work.

    ptc-53490401-Visitor
    1-Visitor
    October 29, 2013

    Hello to anyone reading this, first post here. I'm not really a user of Mathcad, but since my current employer has it installed, I've been giving it a look of late. A few days ago I was playing with the worksheet enclosed in this thread, when Mathcad crashed and I lost all the edits made by then (but tech discussion should better take place in another section, so I'll cut short to what's related to the problem itself).

    For a problem of this kind I would prefer to see the solution in a form similar to as shown below (with text output and equations not much more complicated than necessary to solve the system):

    M1 = F * (1 – x1g / x6g) / ((1 – x3g / x6g) * x7a / x3a – 1)

    M3 = M1 * x7a / x3a

    M6 = M1 + F – M3

    x3s = x1s * F / M3

    x3w = 1 – x3a – x3g – x3s

    M5 = S – M1

    x2w = x5w * M5 / S

    x2a = 1 – x2w

    M4 = M5 + M6

    x4a = x5a * M5 / M4

    x4g = x6g * M6 / M4

    x4w = 1 – x4a – x4g

    Now (the question to PTC support staff and/or experienced users), do I get it right that this is something Mathcad can't do? I mean, when I try sym solution, I'm getting half a page of pretty heavy equations instead of neat formulae as shown above and I don't see how to get it in text. Am I missing some clues?

    And one more thing (to the topic starter): is this a textbook problem, and if so, can you say what you were given (what did you have to work from)? Math is ok, but chemistry (considering L/L extraction terms from your worksheet) doesn't seem to make sense.

    Werner_E
    Werner_E25-Diamond I
    25-Diamond I
    October 29, 2013

    If I buy a software to solve a syszem of equations symbolically and the result would be that one solved for variable is referenced on the RHS of the "solution" of another as is the case in your proagated "solution" multiple times, I'd like my money back!

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