Problem when using "solve" command

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letoppina
letoppina 2018-7-17
评论： Stephan 2018-7-17
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??

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回答（1 个）

Stephan
Stephan 2018-7-17
编辑：Stephan 2018-7-17
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
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letoppina
letoppina 2018-7-17
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
Stephan 2018-7-17
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

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