Why is my fimplicit plot empty?

Question

Rasandemusen 2019-10-10

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/484584-why-is-my-fimplicit-plot-empty

评论： Stephan 2019-10-10

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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Answer 1

Stephan 2019-10-10

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/484584-why-is-my-fimplicit-plot-empty#answer_395702

编辑：Stephan 2019-10-10

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;