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05-19-2020
12:55 PM

05-19-2020
12:55 PM

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

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)

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10 REPLIES 10

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05-19-2020
04:14 PM

05-19-2020
04:14 PM

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

You did not set all the initial conditions.

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05-20-2020
05:57 AM

05-20-2020
05:57 AM

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

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.

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05-20-2020
12:22 PM

05-20-2020
12:22 PM

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

Sorry!

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

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05-20-2020
01:05 PM

05-20-2020
01:05 PM

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

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

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05-20-2020
01:14 PM

05-20-2020
01:14 PM

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

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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05-20-2020
01:45 PM

05-20-2020
01:45 PM

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

Check please your equation with = and see an error!

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05-20-2020
01:59 PM

05-20-2020
01:59 PM

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

i think that it is before i copy those variables. I forgot to upload the new .xmcd.

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05-20-2020
01:52 PM

05-20-2020
01:52 PM

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

M*x=v

M:=[] v:=[]

cols(M)=n1

rank(M)=n2 rank(augment(M, v))=n3

n1=n2=n3!!!

x:=lsolve(M, v)=

M*x-v=

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05-20-2020
02:09 PM

05-20-2020
02:09 PM

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

Is that how to solve this system with lsolve?

I have seen something equal to that method dividing the matrix in 4 and then using:

M = stack(augment(M00,M01),augment(M10,M11))

The problem is that there are some variables that multiply other (x8*h31), making a non linear system, or am i wrong?

That makes impossible using matrices using a column for each variable.

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05-21-2020
01:40 AM

05-21-2020
01:40 AM

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

Non linear.