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rsulwinski1-Visitor
1-Visitor
January 10, 2017
Solved

Incorrect derivative

  • January 10, 2017
  • 16 replies
  • 7366 views

Hello,

I encountered a very strange problem in Mathcad 15. When I want to calculate a derivative symbolically that contains a natural logarithm the solution is wrong. I do not know why is it so.

Attached you can find the Mathcad file.

Regards

Rafal

Best answer by Werner_E

I can confirm that this is  a severe bug.

Here in Mathcad 15 M045

and here in Prime 3.0

As you can see the problem occurs with other functions, too. If the function simplifies to an exact value like ln(e) or if we force to switch to float mode or if we introduce another variable (anywhere) the derivative is OK.

I am not absolutely sure but can it be that we had already discussed this bug quite a while ago? But I could not find an appropriate thread, so maybe I am wrong about that.

Anybody using older releases of Mathcad 15 or even Mathcad 15 and can give it try to see if its a newly introduced bug ar an older one.

16 replies

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
January 10, 2017

Works if you get the syntax right

R
rsulwinski1-VisitorAuthor
1-Visitor
January 10, 2017

Hmm. It still does not work for me. Maybe it is related to Mathcad 15 version/patch? I attached a snapshot of my original problem (left) and your solution (right).

M
MJG1-Visitor
1-Visitor
January 10, 2017

Seems like a bug to me.  You get the correct result if you put variables in the natural logarithm instead of numbers:

R
rsulwinski1-VisitorAuthor
1-Visitor
January 11, 2017

Yes, it is a bug. The problem does not appear in Mathcad version 13.

W
Werner_E25-Diamond IAnswer
25-Diamond I
January 11, 2017

I can confirm that this is  a severe bug.

Here in Mathcad 15 M045

and here in Prime 3.0

As you can see the problem occurs with other functions, too. If the function simplifies to an exact value like ln(e) or if we force to switch to float mode or if we introduce another variable (anywhere) the derivative is OK.

I am not absolutely sure but can it be that we had already discussed this bug quite a while ago? But I could not find an appropriate thread, so maybe I am wrong about that.

Anybody using older releases of Mathcad 15 or even Mathcad 15 and can give it try to see if its a newly introduced bug ar an older one.

A
AlanStevens19-Tanzanite
19-Tanzanite
January 11, 2017

The bug is there in the earliest M15.

If you click on the ln(2)(x)  the display changes to ln(2(x))  

Alan

W
Werner_E25-Diamond I
25-Diamond I
January 11, 2017

AlanStevens wrote:

The bug is there in the earliest M15.

If you click on the ln(2)(x)  the display changes to ln(2(x))

Alan

Oh, I see. Its a strange bug. And when we assign the derivative to a function and evaluate it symbolically, we get a strange error message

In my last sentence above I wanted to write "Anybody using older releases of Mathcad 15 or even Mathcad 14..."

It seems the change quite a bit from 14 to 15 concerning the symbolic calculus (mainly wrong integral tables, I guess) - so maybe it happened there.

V
ValeryOchkov24-Ruby IV
24-Ruby IV
January 11, 2017

I ask my students what is a derivative of x^n.

99% answer n*x^(n-1). And only 1% asks - what is a variable

W
Werner_E25-Diamond I
25-Diamond I
January 11, 2017

????????????

The variable of differentiation in Rafals sheet is clearly dx_M.xx - no doubt.

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