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How can I solve This system of ODEs?

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Jaser Adosari
Jaser Adosari on 1 Dec 2017
Answered: Karan Gill on 1 Dec 2017
I tried to solve these ODE equations but there is an error!
This is my code:
syms x1(w) x2(w) x3(w)
T=773.15; R=1.314;
K=(3.35e+24)*exp(-(70e+3)/(1.987*T));
Keq=exp(53.47-(27330/T));
ode1=diff(x1,w)==((-K*x1)/((x1+x2+x3)*(R*T))*(1-((x2*x3^3)/(x1*((x1+x2+x3)^3)*Keq))));
ode2=diff(x2,w)==((K*x1)/((x1+x2+x3)*(R*T))*(1-((x2*x3^3)/(x1*((x1+x2+x3)^3)*Keq))));
ode3=diff(x3,w)==((3*K*x1)/((x1+x2+x3)*(R*T))*(1-((x2*x3^3)/(x1*((x1+x2+x3)^3)*Keq))));
odes=[ode1;ode2;ode3]
cond1=x1(0)==2;
cond2=x2(0)==0;
cond3=x3(0)==4;
conds=[cond1;cond2;cond3];
[x1Sol(w), x2Sol(w), x3Sol(w)] = dsolve(odes,conds)
And this is the error message
Warning: Explicit solution could not be found.
> In dsolve (line 201)
Error using sym/subsindex (line 796)
Invalid indexing or function definition. When defining a function, ensure that the arguments are symbolic variables and
the body of the function is a SYM expression. When indexing, the input must be numeric, logical, or ':'.

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Answers (2)

Star Strider
Star Strider on 1 Dec 2017
You can create a function usable in the numeric ODE solvers by adding:
[odes_vf, vfsubs] = odeToVectorField(odes);
odes_fcn = matlabFunction(odes_vf, 'Vars',{w, Y});
[w,y] = ode15s(odes_fcn, [0 1], [2; 0; 4]);
just after your ‘odes’ assignment. However when I tried that, the numerical integration threw warnings and produced nothing.

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Karan Gill
Karan Gill on 1 Dec 2017
You have numeric values in your equations, so solve this numerically. Try ode45: https://www.mathworks.com/help/matlab/ref/ode45.html
For the difference between symbolic and numeric methods, see: https://www.mathworks.com/help/symbolic/choose-symbolic-or-numeric-arithmetic.html.
The error is because you didn't declare the symbolic functions [x1Sol(w), x2Sol(w), x3Sol(w)]. You can ignore it for the moment and focus on getting a solution.

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