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R
Rafael11-Visitor
1-Visitor
December 2, 2018
Solved

Solving with several unknown variables

  • December 2, 2018
  • 3 replies
  • 7161 views

Why can't MathCAD solve this equation? It obviously works after simplifying it manually first as shown in the last step. But how can i get MathCAD to do it in the previous step?

Best answer by LucMeekes

Ah, I think I see the problem. You're trying to have the symbolic solver find m.Rd and you are using units. You should be aware that the symbolic solver does not know units, it treats each unit as an undefined variable. Prime's numeric processor knows, and can transform units as required, but the symbolic solver does nothing with/to them  In some cases, depending on the equation, they will cancel out, and then it does not hinder the symbolic processor...

 

Attached is a different approach, using the root function (Prime 4)

Success!
Luc

3 replies

L
LucMeekes23-Emerald IV
23-Emerald IV
December 2, 2018

The problem might occur because you are using a variable named delta, that is undefined. Mathcad assumes that it's the delta function, which requires an argument...

 

You can change delta to be a variable, by setting its label (see the ribbon) accordingly.

 

Success!
Luc

 

 

R
Rafael11-VisitorAuthor
1-Visitor
December 2, 2018

I have tried changing delta to variable, replacing it with an other letter and even defining it as "1".

But nothing works.

L
LucMeekes23-Emerald IV
23-Emerald IV
December 2, 2018

What is the error message that you get from the first symbolic 'solve'?

L
LucMeekes23-Emerald IV
23-Emerald IV
December 2, 2018

Other than that, your equation is indeed simple. Here's how Mathcad can solve it fully symbolically:

LM_20181202_Problem.png

Note that delta is not part of the solution, so can be any value that does not lead to (other) problems.

Success!
Luc

 

W
Werner_E25-Diamond I
25-Diamond I
December 2, 2018

Your problem is basically that you define variables using an unknown, then you solve symbolically for this unknown and then you expect all those defined variables magically have their appropriate value based on the now solved for variable.

Thats not the way Mathcad works. It will never go back to a variable definition and insert a value for a variable which was calculated or assigned later. If you want this to be done, you have to define a function instead of a variable.

Her is a simplified version of your problem as I understand it

B0.png

and her a three possible solutions/work-arounds:

1) Simply assign the solved for value to the appropriate variable and repeat/copy the variable definitions which will happily use the calculated value now.

B1.png

2) Turn your variable definitions into functions as this is the mathematical construct which changes its value depending on one ore more input arguments

B2.png

3) Do without the symbolic solve and use a numeric solve block instead. Note the picture below is erratically missing a guess value for x (we don't get an error because x was already assigned a value before in my sheet)!. Note that you have to "find" all variables which depend on the unknown, even though you might not be interested in all of them

B3.png

 

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