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NovaStark4-Participant
4-Participant
June 14, 2017
Solved

prime 3.1 solve error

  • June 14, 2017
  • 2 replies
  • 2855 views

I am currently using Prime 3.1 to solve a simple equation as given in my mathcad sheet.

My defined equation works to return correct values with the correct units. However using it to solve the equation for a single variable returns that there are no solutions.

Yet I should be able to get back a solution around 950 (verified by graphing in excel).

I am not too used to using this solve function in prime as usually I'd use the solve in Mathcad 15. Can someone more experienced just double check to see f I am doing anything incorrect ?

Best answer by Werner_E

Guess I just found a way to open P31 files in P30 (at least partially)

From what I see you first define a constant sigma.UTS and later you overwrite that value with a function of the very same name.

The function definitely should have a different name (I just added an underscore).

When I plot your function I see that there is a solution around 100 Delta°F, not 950

So maybe you used a different function in Excel.

While the symbolics still would not find an exact solution, the numeric root command does its job without problems:

Guess a solve block with "find" would do the job as well.

2 replies

W
Werner_E25-Diamond I
25-Diamond I
June 14, 2017

Can you post a screenshot / pdf print of your file.

Am limited to P30 at the moment and so can't open your P31 file.

W
Werner_E25-Diamond IAnswer
25-Diamond I
June 14, 2017

Guess I just found a way to open P31 files in P30 (at least partially)

From what I see you first define a constant sigma.UTS and later you overwrite that value with a function of the very same name.

The function definitely should have a different name (I just added an underscore).

When I plot your function I see that there is a solution around 100 Delta°F, not 950

So maybe you used a different function in Excel.

While the symbolics still would not find an exact solution, the numeric root command does its job without problems:

Guess a solve block with "find" would do the job as well.

N
NovaStark4-ParticipantAuthor
4-Participant
June 14, 2017

Thanks!

I think you solved my error.

I made a mistake in the sheet, the 950 referred to solving for Srup (26 ksi or so).

Does this mean that for all functions f(x) = constant, I should use the root function rather than solve ?

W
Werner_E25-Diamond I
25-Diamond I
June 14, 2017

The symbolic solver has its merits when it comes to solving equations with unknown variables symbolically. But it also had its limits. While it would normally switch to a float/numerical mode when it does not find an exact symbolic solution, it did not in case of your equation. Not sure why - it may have to do with the units as the symbolics is in no way unit aware. so ksi and Delta°F are treated as unknown variables.

When I remove the units and just solve for exponent=0 I get the five numerical solution of this fith order polynomial and one of them ist the 96.42 which we found via the root command.

So I would suggest that as long as you are just after a numerical solution you use a numerical method like root or a solve block with Find.

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