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Symbol evaluation (move to Electrical Engineering)

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Contributor

Symbol evaluation (move to Electrical Engineering)

Hi

 

I post this question before but I think I did not label Electrical Engineering but Knowledge base instead. I moving this question to Electrical Engineering

 

Question: Is there a way to substitute a symbol into a symbolic question to simplify the symbolic equation further?

 

symbolic_subs.PNG

 

Thanks a lot for all the helps

rdliquid

1 ACCEPTED SOLUTION

Accepted Solutions
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Re: Symbol evaluation (move to Electrical Engineering)

Thanks to Werner_E and Mfra for the helps. I get it now.

 

Rqliquid

 
 

View solution in original post

8 REPLIES 8
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Re: Symbol evaluation (move to Electrical Engineering)

Hi,

answer to rdliquid.jpg

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Re: Symbol evaluation (move to Electrical Engineering)

One way to do it is to provide the equations not using the assignment operator (:=) but rather with the fat boolean equal sign and let Prime solve each equation for the variable you would like to be substituted. Of course the results would have to be assigned the appropriate variables.

Werner_E_0-1600506650510.png

 

As the equations are so simple it probably would be less effort to manually solve the equations in your head and simply provide the results as MFra also had done:

Werner_E_1-1600506685336.png

 

Prime 6 worksheet attached

 

BTW, the labels you apply to your posts don't really matter. I think these labels are hardly noticed by anyone.

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Re: Symbol evaluation (move to Electrical Engineering)

Thanks to Werner_E and Mfra for the helps. I get it now.

 

Rqliquid

 
 

View solution in original post

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Re: Symbol evaluation (move to Electrical Engineering)

Sorry I have a little of  problem when I did it the way Werner_E recommended. Technical it is correct but can be reduce further. Please see below:

I do not know the difference between the two and yet it gives 2 different results:

symbolic_substitution.PNG

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Re: Symbol evaluation (move to Electrical Engineering)

The problem is due to Primes annoying auto-labelling errors.

Look at the variable c.5134 in the definition of R.l and in the expression you'd like to be simplified. They both must have the same label. In your version, the label in R.l is "(-)", meaning automatic and in the larger expression its "Variable". Change manually either label to match the other and it will work.

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Re: Symbol evaluation (move to Electrical Engineering)

Hi Werner_E,

 

Sorry but I do not know what you mean by "-" in the definition of my R.l. I do not see it

I looked at yours and my definition of variables, they look identical, the only difference is "-" between the

denominator and the numerator, my is darker and your is much lighter. Do you know why it is like that?

 

Thanks for your helps

Rdliquid

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Re: Symbol evaluation (move to Electrical Engineering)

I was talking about the "label" a name can have. The options are "Variable", "Unit", "Constant", "Function", "System", "Keyword" and (actually the first in the list) "Automatic". The latter is denoted by a dash "-".

 

Werner_E_0-1600616172456.png

See  http://support.ptc.com/help/mathcad/en/index.html#page/PTC_Mathcad_Help/about_labels.html

 

Prime tries to assign the appropriate label when you type along, but ever so often fails - especially when it comes to symbolic evaluations (the symbolic doesn't know anything about units and labels).

There is an calculation option one can turn on to support units and constants in symbolic calculation. Sometimes it helps - ever so often it doesn't.

 

So click on the variables and look at how they are labelled. You may change the label via the menu or the keyboard shortcut Ctrl-Q.

The important thing seems to be that both must have the same label (either both "-"/"Automatic" or both "Variable"). Otherwise the symbolic engine treats them as two different variables and so they don't cancel.

 

I just see that all variables are labelled "-" and only c.1 and c.5134 are labelled "Variable" when you define R.s and R.l. Not sure why this happened - it may happen when you edit/copy an existing expression a few times, I guess. When I type the assignment from scratch, all variables are labelled "-".

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Re: Symbol evaluation (move to Electrical Engineering)

Hi Werner_E,

 

Thanks a lot for all your helps. I did not know there is such a feature like that in Mathcad 6.0

I got it now and was able to get it to work

 

Thanks!

Rdliquid

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