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I
IG881-Visitor
1-Visitor
May 19, 2020
Question

Problem with given/find in linear system. "This variable is undefined"

  • May 19, 2020
  • 2 replies
  • 3400 views

Hello,

 

I am doing this exercise about a combined cycle where some of the enthalpies are known. 

The problem is when i try to solve the system the error "This variable is undefined" constantly appears.

I have tried both of the systems i know about describing the solution: 

 

 Solution:=Find(x1,...) and (x)

                                       (y)  :=Find(x1,...)

 

Hope there is solution.

Thanks in advance.

IG88

 

(Uploaded the .xmcd)

 

2 replies

V
24-Ruby III
VladimirN24-Ruby III
24-Ruby III
May 19, 2020

You did not set all the initial conditions.

    I
    IG881-VisitorAuthor
    1-Visitor
    May 20, 2020

    Yes, you are completely right. I forgot to copy those variables. Thanks @VladimirN 

     

    It shows me the same error with Find and i have tried to use Minerr but shows nonsense results, knowing the differences between them, so i will continue finding a solution.

     

     

     

    V
    24-Ruby IV
    ValeryOchkov24-Ruby IV
    24-Ruby IV
    May 20, 2020

    Sorry!

    Better to use the lsolve function for this problem! Not given-find!

     

    L
    LucMeekes23-Emerald IV
    23-Emerald IV
    May 20, 2020

    I think I have to disagree with you, Valery.

    Using lsolve you are forced to structure your problem nicely. The advantag of a solve block is that you can make a mess of the problem.

     

    Luc

    I
    IG881-VisitorAuthor
    1-Visitor
    May 20, 2020

    The main problem i have just realised is that in some of the equations some unknown variables are multiplying each other, such as x8 and h31. I thought that the idea of using lsolve was great until i realised about that.

     

    I would change the equations and assign values for the h24, h31 and h32 so there isn´t any variable multiplying another.

     

    Thanks.

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