vpasolve gives wrong solution

Question

Alessio Falcone 2021-10-21

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1568843-vpasolve-gives-wrong-solution

回答： nick 2024-5-9

Hi, I'm Alessio and I'm a chemical engineer. I have basic knowlwdge of matlab and I have some problems with a system of equations that I can't manage to solve. Could you please help me ?

Tc=200;
Pc=45;
R=0.0821;
Z=0.30;
syms Vc a b c;
EoS = (R*Tc)/(Vc-b)-(a-c*sqrt(Tc))/(Vc*(Vc+b)+b*(Vc-b)) - Pc;
eq1 = (R*Tc)/(Vc-b)-(a-c*sqrt(Tc))/(Vc*(Vc+b)+b*(Vc-b)) - Pc == 0;
eq2 = diff (EoS, Vc) == 0;
eq3 = diff (EoS,Vc, 2) == 0;
eq4=(Pc*Vc)/(R*Tc)-Z==0;
sol = vpasolve([eq1,eq2,eq3,eq4],[a,b,c,Vc]);
Vc = sol.Vc
Vc = 0.10946666666666666666666666666667
b = sol.b
b = 0.26427591129444147134215819234349
a = sol.a
a = 0
c = sol.c
c = 0

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Srijith Kasaragod 2021-12-2

编辑：Srijith Kasaragod 2021-12-2

Could you please share what issues you are facing? In case you are facing any errors, please share the error message.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

nick 2024-5-9

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1568843-vpasolve-gives-wrong-solution#answer_1455031

Hi Alessio,

As Srijith mentioned kindly share the error message and the issue faced by you. Since the code runs without any error, I assume that you are referring to the solutions of a,b,c,Vc not having the desired values as the problem.

This is likely to be the case because the system of equations has more than one solution. If no initial guess is mentioned using the "init_param" argument, the "vpasolve" function returns the first solution it finds.

You may refer to the following documentation to learn more about "vpasolve" :

https://www.mathworks.com/help/symbolic/sym.vpasolve.html

Hope this helps.