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Why is my fimplicit plot empty?

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Rasandemusen
Rasandemusen on 10 Oct 2019
Commented: Stephan on 10 Oct 2019
syms x(t)
ode45 = diff(x,t,2) + ((4.2/(t + 3))/2)*x == 0;
Dx = diff(x);
DDx = diff(Dx);
cond1 = x(0) == 0;
cond2 = Dx(0) == 5;
conds = [cond1 cond2];
xSol = dsolve(ode45,conds);
fimplicit (@(t) xSol(t),[-20 20 -20 20])
xlim([-10 10]);
ylim([-10 10]);
grid on;
I am trying to plot this equation, the plot opens up but its all empty.
Warnings: Unable to find explicit solution.
> In dsolve (line 201)
In test (line 12)

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Accepted Answer

Stephan
Stephan on 10 Oct 2019
Edited: Stephan on 10 Oct 2019
The warning means, that Matlab did not find an analytical solution - solve numeric:
syms x(t) Dxt(t)
ode = diff(x,t,2) + ((4.2/(t + 3))/2)*x == 0;
[ode, vars] = reduceDifferentialOrder(ode, x);
[V, S] = odeToVectorField(ode);
fun = matlabFunction(V,'Vars',{'t','Y'});
[t, xSol] = ode45(fun,[0 200],[0 5]);
plot(t,xSol)
grid on;

  2 Comments

Rasandemusen
Rasandemusen on 10 Oct 2019
One more thing, does the orange curve show the derivate of the blu function?
Thank you for the help!
Stephan
Stephan on 10 Oct 2019
Yes, the order of the variables is saved in vector S. in this order Y[1] and Y[2] are saved in the functionHandle.

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