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regcurry
regcurry16-Pearl
16-Pearl
October 19, 2026
Solved

Large matrix in solve block i Mathcad 15

Attached is a zip file containing a large matrix in a solve block.

There are two questions in yellow boxes.  Any help is appreciated.

The jpg file shows the geometry of the diffusion equation problem.

terryhendicottAnswer
21-Topaz II
October 19, 2026

Three Problems

Solve block needs to be a matrix equation the way you have set it up.  I would write them as five separate equations?

Problem two is that the matrix elements have mixed units and this is not permitted in a Mathcad 15 matrix (It is in Prime).  Two rows are unitless. Three rows are 1//m.  See enclosed file

Third five equations in a solve block you would normally search for five unknowns not one?

 

Cheers

Terry

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3 replies

terryhendicott21-Topaz IIAnswer
21-Topaz II
October 19, 2026

Three Problems

Solve block needs to be a matrix equation the way you have set it up.  I would write them as five separate equations?

Problem two is that the matrix elements have mixed units and this is not permitted in a Mathcad 15 matrix (It is in Prime).  Two rows are unitless. Three rows are 1//m.  See enclosed file

Third five equations in a solve block you would normally search for five unknowns not one?

 

Cheers

Terry

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    regcurry
    regcurry16-PearlAuthor
    16-Pearl
    October 19, 2026

    Terry,

    Thanks much. You are always a big help.

    The objective is to eliminate the unknown coefficients C, A,B,D,G and use the resulting transcendental equation to fine the values of 𝛼 that satisfy the transcendental equation.

    You helped me 4 months ago with a similar problem for 2 layers and three unknown coefficients instead of 3 layers and five unknown coefficients.

    https://community.ptc.com/mathcad-7/help-with-mathcad-15-solve-block-174320

    Although you had everything unitless, when I used your formalism in my worksheet, I included 1/lenth for the 𝜆s and length for the Rs. That made K1 having  units and K2 no units, I think.🤞  The solve block worked.

    So, if I write it as five separate equations, does that mean every element in each equation can have both units and no units?

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    regcurry
    regcurry16-PearlAuthor
    16-Pearl
    October 19, 2026

    Mea culpa 

    ‘The 2 layer equations in the solve block are unitless.
     

    Also, the matrix approach to arriving at the correct transcendental equation to solve for 𝛼  in the solve block for the three layer problem is incorrect.  It must be done the way it was done in the 2 layer problem.  It’s a lot more algebra for the three layer problem.  I am using MATHCAD’s symbolic solve→ and simplify→ functions as I did for the 2 layer problem.  When I get it working, I will post it for those with natural curiosity.

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