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SP_97819697-Bedrock
7-Bedrock
March 9, 2023
Question

Mathcad fails to provide positive results

  • March 9, 2023
  • 1 reply
  • 1695 views

Mathcad 15 does not make assumptions in very simple problem for find function and keyword solve to provide only positive results .

 

SP_9781969_0-1678400577672.png

 

1 reply

L
LucMeekes23-Emerald IV
23-Emerald IV
March 9, 2023

So:

LucMeekes_0-1678402633462.png

means there is ONE solution, where x=-1/2 and y=7/2.

If you require that both x AND y are positive, Mathcad answers:

LucMeekes_1-1678402722268.png

I think that's correct.

Success!

Luc

W
Werner_E25-Diamond I
25-Diamond I
March 10, 2023
@LucMeekes wrote:

So:

LucMeekes_0-1678402633462.png

means there is ONE solution, where x=-1/2 and y=7/2.

If you require that both x AND y are positive, Mathcad answers:

LucMeekes_1-1678402722268.png

I think that's correct.

Success!

Luc

No solution found definitely IS correct.

But what you show is from Mathcad 11 with Maple under the hood.

 

Mathcad 15 with muPad as symbolic engine actually ignores (as so often) the "assume" modifier or the additional constraints in the solve block and returns a solution not meeting the constraints.

Werner_E_0-1678411330263.png

Evaluating the solve block numerically would respect the extra conditions

Werner_E_1-1678411426539.png

As usual with solve blocks the error message "This variable is undefined." is misleading and not helpful.

 

At least MC15 seems to work OK if there actually exists a solution meeting the extra conditions

Werner_E_2-1678411725007.png

 

 

 

S
SP_97819697-BedrockAuthor
7-Bedrock
March 15, 2023

That makes perfect sense. So apart from extra conditions there is no other keyword to meet the expectation ?

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