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Aug 21, 2018
09:44 AM

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Aug 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.

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8 REPLIES 8

Aug 21, 2018
10:51 AM

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Aug 21, 2018
10:51 AM

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

Aug 21, 2018
04:16 PM

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Aug 21, 2018
04:16 PM

Thanks!

It works.

I though I should not include n.

Aug 21, 2018
07:13 PM

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Aug 21, 2018
07:13 PM

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.

Aug 22, 2018
06:21 AM

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Aug 22, 2018
06:21 AM

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.

Aug 22, 2018
08:25 AM

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Aug 22, 2018
08:25 AM

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.

Aug 21, 2018
04:27 PM

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Aug 21, 2018
04:27 PM

deleted.

Aug 21, 2018
04:29 PM

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Aug 21, 2018
04:29 PM

Thanks!

I thought I should not put n.

it works now.

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

Aug 21, 2018
01:21 PM

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Aug 21, 2018
01:21 PM

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

Delete it!