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08-21-2018
09:44 AM

08-21-2018
09:44 AM

Variable in Mathcad and Symbollic Differentiation

Hi,

I would like to get the expression of a function after differentiation with respect to a variable.

Somehow Mathcad does not recognize the variable and tells that the variable is undefined.

it leads to make it impossible to find the derivative.

numerically, the calculation is correct.

It is just this symbolic feature that are having problem.

I appreciate any suggestion.

8 REPLIES 8

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08-21-2018
10:51 AM

08-21-2018
10:51 AM

Re: Variable in Mathcad and Symbollic Differentiation

The red circle is one problem, but it shouldn't stop you from getting symbolic derivative:

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08-21-2018
04:16 PM

08-21-2018
04:16 PM

Re: Variable in Mathcad and Symbollic Differentiation

Thanks!

It works.

I though I should not include n.

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08-21-2018
07:13 PM

08-21-2018
07:13 PM

Re: Variable in Mathcad and Symbollic Differentiation

Either

or

The first is the cleaner version.

In the latter case you will get an error by the numerics because n is not defined, but the symbolic evaluations should work nonetheless.

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08-22-2018
06:21 AM

08-22-2018
06:21 AM

Re: Variable in Mathcad and Symbollic Differentiation

Thank you.

I can fix some errors and understand a little bit how to deal with functions and labels.

But I still have more doubts about labels.

In the picture below, Rpu in the denominator in the last equation is labeled as variable, but it still has some errors.

If I copy only the equation into a new sheet/ new file,it works most of the time.

If I copy the equation with other equation before it (the ones at the top), the same problem happens.

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08-22-2018
08:25 AM

08-22-2018
08:25 AM

Re: Variable in Mathcad and Symbollic Differentiation

I don't see any errors in your sheet or your file.

Your problem seems not to be one of labeling but that you don't understand, that you have to provide arguments when you are using a function.

If you just use the function name without the correct number of arguments. Mathcad will just tell you that is a function.

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08-21-2018
04:27 PM

08-21-2018
04:27 PM

Re: Variable in Mathcad and Symbollic Differentiation

deleted.

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08-21-2018
04:29 PM

08-21-2018
04:29 PM

Re: Variable in Mathcad and Symbollic Differentiation

Thanks!

I thought I should not put n.

it works now.

one question, could you tell me what "function" in the result means?

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08-21-2018
01:21 PM

08-21-2018
01:21 PM

Re: Variable in Mathcad and Symbollic Differentiation

The problem is the inline evaluation (the last equal sign) when you define your functions.

Delete it!