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3-Visitor

## Non-linear equations system

Trying to solve a systems of 3 equations with 3 unknowns by means of SOLVE BLOCK and MINERR function. (or find). It doesn't provide any error message but it seems not to calculate any root; it simply provides the three guessed values as the results without running any iteration.

8 REPLIES 8
21-Topaz I
(To:PB_9907889)

Hi,

Cheers

Terry

Hi,

Here the file and the solved example from a book.

Thanks a lot!

21-Topaz I
(To:PB_9907889)

Hi,

I have carefully checked your work, It is correct.

I also get the wrong answer compared to the original.

I trust someone from PTC is watching this thread and can see the errors in Prime.

Cheers

Terry

23-Emerald III
(To:PB_9907889)

Mathcad 11 apparently (also) can/will not solve this set.

The PDF appears to show a Mathcad original sheet (part), but it's fake.

This:

a square bracket opening, closing with a round bracket.

should never happen in a mathcad expression.

Consider that the first expression doesn't depend on p (actually, it's rho). When running over all values of alpha and beta I find a near equality close to alpha=0.9 and beta=0.3, independently of p of course.

I squared both sides of this equation and subtracted, instead of requesting equality, in order to find a zero-crossing.

That's close to 0 but nowhere near the solution that the PDF presents.

At alpha=beta=1.8, the value becomes

Further: The second equation has

on both sides of the equals sign. That's silly. Besides I haven't seen it produce other than equality for any value of alpha, beta and p that I tried. Maybe it's functionally independent of all three variables...

The third equation even has this same factor multiplied with itself, and twice divided by itself.

Consider that the picture was edited after it was calculated, and now contains serious mistakes.

edit: Oh, I now see that the original sheet used Minerr, NOT Find...

Success
Luc

P.S. This thread does not belong in this part of the forum, since it has nothing to do with installation of Mathcad or Prime. It should be in this list: https://community.ptc.com/t5/PTC-Mathcad/bd-p/PTCMathcad

23-Emerald III
(To:PB_9907889)

See if you can get it to converge with guess values:

alpha=0.98, beta=0.937, p=31.25

Final note: In the third equation there's an undefined R. You (and I) assumed that it's equal to r, but that is not necessarily true; although it is probably unlikely that R is the universal gas constant...

It is one more indication that the sheet as presented would have never calculated without error.

Success!
Luc

23-Emerald III
(To:PB_9907889)
3-Visitor
(To:LucMeekes)

Thanks a lot for you great help!

Despite the example has taken from a technical manual, you're right that equations have been manually modified...

Unfortunately I've already tried  with these new set of equations (i.e. for stiffened liners) but I get almost the same results; not convengerce or imaginary solutions.

17-Peridot
(To:PB_9907889)

I watched your conversation. I've checked the sample and I do confirm your findings.

From PTC perspective, Mathcad Prime works to product specification. Means, the Solve block as shown cannot be solved. There are obviously several typos and/or other mistakes.

It is a good approach to counter check it against any other manual for reference.

Once fixed, it should work.

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