MathCAD 15 does not want to solve

Forum|Forum|10 years ago
May 9, 2016
3 replies
6944 views

It should be fairly simple, y = 36 cm

However, MathCAD cannot solve it, how come?

1_problem-xmcd.zip

Other

Best answer by Werner_E

It should be fairly simple, y = 36 cm

Its not that simple and 36cm is not the exact solution! But Mathcad symbolics is here to give exact solutions and the exact solution in your example is the solution of a fifth order polynomial. There is no general mathematical algebraic solution to this problem and thats the reason Mathcads symbolic gives you this correct, but rather useless answer.

Furthermore the symbolics does not know anything about units, it does not know if m may be negative ore even imaginary and it does not know that 1L equals 10^-3 m^3.

For your task its better to use a numeric approach to get the answer you expect.

This can be a solve block

or you may use the "root" function with a guess value

or you let the "root" function search for a solution within a given interval without providing a guess

Werner

W

Werner_E

Answer

25-Diamond I

It should be fairly simple, y = 36 cm

Its not that simple and 36cm is not the exact solution! But Mathcad symbolics is here to give exact solutions and the exact solution in your example is the solution of a fifth order polynomial. There is no general mathematical algebraic solution to this problem and thats the reason Mathcads symbolic gives you this correct, but rather useless answer.

Furthermore the symbolics does not know anything about units, it does not know if m may be negative ore even imaginary and it does not know that 1L equals 10^-3 m^3.

For your task its better to use a numeric approach to get the answer you expect.

This can be a solve block

or you may use the "root" function with a guess value

or you let the "root" function search for a solution within a given interval without providing a guess

Werner

1_problem_WE160409-xmcdz.zip

L

LucMeekes

23-Emerald IV

First off, the equation where you try to solve for y does not contain y. Even though you define A and P.eri in y, because y is undefined, A end p.eri are undefined and the symbolic processor then treats those (A and p.eri) as (un)known variables.

To get a sound equation to solve for y would be e.g.:

Secondly, the symbolic processor does not deal with units. The L/s appended to 200 are treated as another set of (un)known variables.

Thirdly, the equation is probably too complicated to solve symbolically. Try numeric:

Success!
Luc

W

Werner_E

25-Diamond I

LucMeekes wrote:

First off, the equation where you try to solve for y does not contain y. Even though you define A and P.eri in y, because y is undefined, A end p.eri are undefined and the symbolic processor then treats those (A and p.eri) as (un)known variables.

No, it doesn't! At least not Mupad in Mathcad15. The result of the symbolics repsects the definitions of A, P.eri, etc. and doesn't treat em as unknown variables.

The main problem as already stated is that Mathcad/Mupad is forced to solve a polynomial equation of fifth order which it can't find a solution for.

Here I tell muPad a little bit about s, m and L and also rewrote I.0 as fraction to avoid muPad to autoswitch in float mode (a very nasty habit of mupad):

Making the problem unitless by defining m:=1 and s:=1 we can even get a numeric solution from the symbolics, but of course using of of the numeric ways shown is preferable