Problem when using "solve" command

Hi,

I have a problem using the command "solve", I want to solve a 3rd order equation as shown below:

syms V R gamma r Qi

eqn = (1+R*0.5*(1-gamma))*V^3 - R*V^2 - R*0.25*gamma*r*V + R*r^(2/3)*gamma*1/(8*Qi) == 0;

V_solve = solve(eqn,V)

when I run the code, everything gets stuck and I have to close Matlab and re-open it. What is the problem??

Answer 1

[Stephan]

Hi,

in your code there is a closing parenthesis missing at the end of the equation - use this:

syms V R gamma r Qi
eqn = (1+R*0.5*(1-gamma)*V^3 - R*V^2 - R*0.25*gamma*r*V + R*r^(2/3)*gamma*1/(8*Qi) == 0);
V_solve = solve(eqn,V)

But this mistake usually will not lead to this kind of problem. Do you use R2017b or R2018a? Then i recommend to update your matlab since there are known problems by using syms in these releases, which are fixed in the updates..

You find it here:

HOME-TAB / Add-Ons (little arrow) / Check for Updates / Products

If you updated your release and fix the syntax error it should be done.

Best regards

Stephan

[letoppina]

Hi Stephan,

thank you for your answer. The closing parentehsis is actually not missing in my original code in matlab (so I have made a mistake when typing it here - I have changed it now). However, I have the R2018a version and by checking for updates it turned out that my version is the lastest one. Do you have other suggestions? Thank you!

[Stephan]

Hi,

since you have an actual release you could use matlab online to calculate your equation. But this is not a solution to your problem... I did this and it works so far without problems.

I also tried your code on my netbook (2x 1,5 GHz @ 4GB RAM @Win10 64 Bit) and it also worked - also with newest update: MATLAB Version: 9.4.0.902940 (R2018a) Update 4:

>> syms V R gamma r Qi

eqn = (1+R*0.5*(1-gamma)*V^3 - R*V^2 - R*0.25*gamma*r*V + R*r^(2/3)*gamma*1/(8*Qi) == 0);
tic
V_solve = solve(eqn,V)
toc

V_solve =

   root(- 8*Qi + 8*Qi*R*z^2 - 4*Qi*R*z^3 - R*gamma*r^(2/3) + 4*Qi*R*gamma*z^3 + 2*Qi*R*gamma*r*z, z, 1)
   root(- 8*Qi + 8*Qi*R*z^2 - 4*Qi*R*z^3 - R*gamma*r^(2/3) + 4*Qi*R*gamma*z^3 + 2*Qi*R*gamma*r*z, z, 2)
   root(- 8*Qi + 8*Qi*R*z^2 - 4*Qi*R*z^3 - R*gamma*r^(2/3) + 4*Qi*R*gamma*z^3 + 2*Qi*R*gamma*r*z, z, 3)

Elapsed time is 2.927402 seconds.
>>

So my suggestion would be to reinstall matlab and/or the symbolic toolbox and if it wont work it should be a case for the support i guess.

Best regards

Stephan

[letoppina]

thank you very much! I have a question: why are the roots in function of "z"? Is it possible to get a solution with the parameters involved only (for example like the expression below)?

-(b + (b^2 - 4*a*c)^(1/2))/(2*a)

[Stephan]

Sorry - in this question i can not help. It seems that symbolic toolbox did a z-transform to solve the problem. But i have really no idea how to deal with this - i think this is worth a new question.

Best regards

Stephan