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jlindrup1-Visitor
1-Visitor
October 26, 2015
Solved

No possible solution Mathcad 15

  • October 26, 2015
  • 13 replies
  • 4672 views

I have MathCad V.15, and theres one equation I can't solve. I've tried a simplified solution which works, but I can't figure whats wrong. Can you please help me?

See attachments.

Thank you.

Best answer by StuartBruff

Jens Lindrup wrote:

I have MathCad V.15, and theres one equation I can't solve. I've tried a simplified solution which works, but I can't figure whats wrong. Can you please help me?

See attachments.

Thank you.

Stuart

(PS. Please don't post the same question multiple times.   Those who are likely to answer will see it no matter what category you post it under, and it can lead to people wasting their time and effort answering a question that's been already been answered elsewhere).

13 replies

S
StuartBruff23-Emerald VAnswer
23-Emerald V
October 26, 2015

Jens Lindrup wrote:

I have MathCad V.15, and theres one equation I can't solve. I've tried a simplified solution which works, but I can't figure whats wrong. Can you please help me?

See attachments.

Thank you.

Stuart

(PS. Please don't post the same question multiple times.   Those who are likely to answer will see it no matter what category you post it under, and it can lead to people wasting their time and effort answering a question that's been already been answered elsewhere).

A
AlanStevens19-Tanzanite
19-Tanzanite
October 26, 2015

Could do it symbolically this way:

integralsolve.PNG

Removing the "assume b = real" gives the other, complex, solutions as well.

Alan

    R
    RichardJ19-Tanzanite
    19-Tanzanite
    October 26, 2015

    Another example of just how poorly the symbolic engine in Mathcad handles definite integrals. It can find the indefinite integral, so you can do the substitution for the limits by hand. In this case even that step is not really necessary though, because the lower limit is 0 and there are no terms that do not contain x.

      S
      StuartBruff23-Emerald V
      23-Emerald V
      October 26, 2015

      Richard Jackson wrote:

      Another example of just how poorly the symbolic engine in Mathcad handles definite integrals. It can find the indefinite integral, so you can do the substitution for the limits by hand. In this case even that step is not really necessary though, because the lower limit is 0 and there are no terms that do not contain x.

      I was wondering how Find managed to symbolically do what it's name suggests if it couldn't evaluate a complex integral?  I now wonder if it's possible that it solves the indefinite integral and then tries the find method?

      Stuart

      R
      RichardJ19-Tanzanite
      19-Tanzanite
      October 26, 2015
      I was wondering how Find managed to symbolically do what it's name suggests if it couldn't evaluate a complex integral?  I now wonder if it's possible that it solves the indefinite integral and then tries the find method?

      

      Stuart

      It's not clear to me what the symbolic processor does when given a solve block with find. Certainly not the same thing it does with the solve keyword.

      W
      Werner_E25-Diamond I
      25-Diamond I
      October 26, 2015

      Hi Jens!

      It should work the way you did it but it doesn't because of a bug obviously introduced with MC14.

      I ran into the very same bug a short while ago -> Bug in Mathcad 15 M030 (symbolics)

      You have already seen some workarounds and I may add another one. As the problem in the symbolics are the floating point decimal numbers, we can rewrite r(x) using fractions of integers. And - Voilà! Mathcads symbolics is capable to solve for all solutions, even though it switches to float display itself:

      If you manually force it in float mode (even after solve), it fails again - clearly a bug!

      Regards, Werner

      S
      StuartBruff23-Emerald V
      23-Emerald V
      October 26, 2015

      Werner Exinger wrote:

      Hi Jens!

      It should work the way you did it but it doesn't because of a bug obviously introduced with MC14.

      I ran into the very same bug a short while ago -> Bug in Mathcad 15 M030 (symbolics)

      You have already seen some workarounds and I may add another one. As the problem in the symbolics are the floating point decimal numbers, we can rewrite r(x) using fractions of integers. And - Voilà! Mathcads symbolics is capable to solve for all solutions, even though it switches to float display itself:

      If you manually force it in float mode (even after solve), it fails again - clearly a bug!

      Thanks,Werner - you've literally just saved me from doing the same thing!   I was idly browsing through the (non-compressed) mpl, lisp and mupad files and wondered whether the symbolic processor might handle a rational solution any differently.

      zsort is a simple sort function I put together to sort complex numbers by their Real values and sort any values with equal Reals by their Imaginary values, simply so I could look at the differences between the solutions..

      Stuart

      M
      MikeArmstrong1-Visitor
      1-Visitor
      October 27, 2015

      Sorry to sound really stupid, but could someone please explain why r(x) is raised is changed to (r(x))^2 in both the examples below.

      Thanks,

      Mike

      S
      StuartBruff23-Emerald V
      23-Emerald V
      October 27, 2015

      Mike Armstrong wrote:

      Sorry to sound really stupid, but could someone please explain why r(x) is raised is changed to (r(x))^2 in both the examples below.

      Thanks,

      Mike

      What do you mean, Mike?  It only seems to have the extra pair of parentheses in the integral that you show?  (The reason it's in my answer is simply that it was that way in the original post; it doesn't affect the results).

      Stuart

      M
      MikeArmstrong1-Visitor
      1-Visitor
      October 27, 2015

      I believe it may be down to my lack of knowledge.

      Mike

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