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bwilson-410-Marble
10-Marble
July 18, 2018
Solved

Two different symbolic solutions...

  • July 18, 2018
  • 2 replies
  • 2291 views

I am getting two different solutions depending on whether I solve indirectly (solve symbolically for the equation first and then plug in variables) or directly (plug variables into the equation and then solve) with the symbolic solver. I have checked using a solve block near the bottom of the sheet and also by hand. I am having trouble recognizing where the the incorrect solution from the direct method is coming from... Is it not good practice to send units through the symbolic solver? Any help is appreciated.

Best answer by LucMeekes

It is indeed NOT good practice to feed units to the symbolic engine of Mathcad. The symbolic engine will treat them as unknown variables, if you're lucky.

If you want to work symbolically, use ONLY symbols for your variables, solve your system, then feed your data (values of variables, with any units) to the symbolic solution.

You can assign the symolic solution to a function, with your variables as parameters. That saves copying the symbolic solution (and making errors in them by copying).

Here's an example:

LM_20180718_Symbolic.png

 

Success!
Luc.

2 replies

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LucMeekes23-Emerald IVAnswer
23-Emerald IV
July 18, 2018

It is indeed NOT good practice to feed units to the symbolic engine of Mathcad. The symbolic engine will treat them as unknown variables, if you're lucky.

If you want to work symbolically, use ONLY symbols for your variables, solve your system, then feed your data (values of variables, with any units) to the symbolic solution.

You can assign the symolic solution to a function, with your variables as parameters. That saves copying the symbolic solution (and making errors in them by copying).

Here's an example:

LM_20180718_Symbolic.png

 

Success!
Luc.

    W
    Werner_E25-Diamond I
    25-Diamond I
    July 18, 2018

    Temperature "units" can be quite tricky and a PITA.

    If you add a numeric evaluation after defining variables with unit °F, you allow Mathcad to not only store the temp in K for the numeric but also for the symbolics and you get the result you expect with both methods.

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