d solve command matlab

5 次查看（过去 30 天）
Kees
Kees 2024-10-4
评论： John D'Errico 2024-10-5
Ran in:
hello i just started programmig with matlab and encountered this error :
Error using symengine (line 58)
Could not extract differential variables to solve for. Use 'solve' or 'vpasolve' to compute the solutions of
non-differential equations.
Error in mupadengine/feval (line 155)
symengine('error',S(8:find(S=='[',1)-2));
Error in dsolve>mupadDsolve (line 328)
T = feval(symengine,'symobj::dsolve',sys,x,options);
Error in dsolve (line 189)
sol = mupadDsolve(args, options);
Error in hwbrownwk202016 (line 6)
solve = simplify(dsolve([diffeq,IC],symvar(V(t))))
this is the program
clear all
syms e P S A t m V0 t0 V(t) real
Warning: Can only make assumptions on variable names, not 'V(t)'.
IC = V(0)==V0
diffeq = diff(V(t),t) == P + e*V(t) - m*(S-(V(t)))
solve = simplify(dsolve([diffeq,IC],symvar(V(t))))
the program work in the online version but not in 2013...anybody knows wat the problem is?
  3 个评论
显示 1更早的评论
Kees
Kees 2024-10-4
hello Rahul, great and thanx! spend hours on this but in the end it was just a syntax difference between the old en new version..
John D'Errico
John D'Errico 2024-10-5
@Kees
Um, you can still use the "old" syntax, as I show how to do so in my answer. It is completely valid in the current release.

请先登录，再进行评论。

请先登录，再回答此问题。

回答（2 个）

John D'Errico
John D'Errico 2024-10-4
编辑：John D'Errico 2024-10-4
Ran in:
syms e P S A t m V0 t0 V(t) real
Warning: Can only make assumptions on variable names, not 'V(t)'.
The point being that assumptions can apply only to variables, not functions. V(t) is a function of t.
IC = V(0)==V0
diffeq = diff(V(t),t) == P + e*V(t) - m*(S-(V(t)))
Next, NEVER name a variable solve!!!!!!!!!
You will be using the FUNCTION solve at aoms point intime. NAming a variable will just have you soon be posting an anguished question, asking why the solve function does not work for you.
Just call simplify with two arguments, and not inside brackets.
Vsol = simplify(dsolve(diffeq,IC))
solve =
(S*m - P + exp(t*(e + m))*(P + V0*e - S*m + V0*m))/(e + m)
  1 个评论
John D'Errico
John D'Errico 2024-10-4
Answers is still not working properly, in terms of displaying results. So I had to paste in the result from my own command window.

请先登录，再进行评论。

Rahul
Rahul 2024-10-4
Ran in:
Hi Kees,
I believe you are trying to solve differential equations for a particular solution, involving symbolic variables and their abstract functions.
In order to use the function “diff” correctly, ‘V(t)’ can be referred to as an arbitrary symbolic function, i.e., a function with no definition. Here, the function's name is ‘V’ (like a function handle) and it is represented by the abstract formula 'V(t)', which just means that it's a function of t. When you want to take the derivative of an abstract function, pass in the name of the function, in your case, ‘V’. While evaluating the function you can use the formula, e.g., V(0), the output of which is a ‘sym’ rather than a ‘symfun’.
Here is how you restructure your code snippet:
clear all
syms e P S A t m V0 t0 V(t) real
Warning: Can only make assumptions on variable names, not 'V(t)'.
IC = V(0)==V0
diffeq = diff(V,t) == P + e*V(t) - m*(S-(V(t)))
solve = simplify(dsolve([diffeq,IC],symvar(V(t))))
To know more about the usage of symbolic differentiation using "diff" function in MATLAB R2013a, you can use the following commands:
help sym/diff
doc sym/diff

类别

在 Help Center 和 File Exchange 中查找有关 Programming 的更多信息

标签

产品


版本

R2013a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by