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Oct 08, 2010
07:54 AM

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Oct 08, 2010
07:54 AM

Problem solving linear equation matrix system

Hi,

I have linear equations in matrix form like A*X=B. (Given are A and B; A is a 15x15, B a vector 15 long)

Of course I want to obtain the solution vector X 😉

I can achieve this in mathcad via lsolve(A,B) or X=(A^-1)*B.

Now here is the problem: if the values of matrix A are calcuated by other matrices inside mathcad I obtain different (wrong) results

for X in comparision to a matrix A already filled with numbers and copied out of excel. I checked the two versions of A and they are identical.

Is there a problem with formats or have I missed something?

Jan

Solved! Go to Solution.

1 ACCEPTED SOLUTION

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Oct 08, 2010
10:16 AM

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Oct 08, 2010
10:16 AM

Need to see a worksheet to be sure, but I bet the two matrices are not the same. They may not be very different, but they are not identical.

4 REPLIES 4

Oct 08, 2010
09:52 AM

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Oct 08, 2010
09:52 AM

Can't tell anything without a worksheet.

TTFN

Oct 08, 2010
10:16 AM

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Oct 08, 2010
10:16 AM

Oct 08, 2010
10:30 AM

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Oct 08, 2010
10:30 AM

Richard, you are of course right. After having a more intensive look at both A matrices I found one number (all others were correct) wich caused the wrong solution.

Thanks very much. I will have a nice weekend now.

Jan

Oct 08, 2010
10:35 AM

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Oct 08, 2010
10:35 AM

Actually, that's not quite what I expected. I thought that maybe your matrix was near singular, in which case the inversion could be very sensitive to roundoff errors. So if you lost any precision when you copied it from Excel you would get a different answer.

Glad it's working though.