![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/346425/image.jpeg)
Solve ODE via Midpoint rule nonlinear system
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Mathematica
le 15 Août 2020
Commenté : Mathematica
le 15 Août 2020
Hey ,
I have the following nonlinear system.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/346392/image.png)
And I'd like to approximate the solution via the midpoint rule , that is
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/346401/image.png)
I choose the stepsize h=0.01. x_{n+1} denotes the solution at time t=h(n+1).
I did the following:
fun=@root2d;
x0=[20,20];
sol=fsolve(@(x)root2d(x,20,20),x0)
function F=root2d(x,xold,yold)
F(1)=x(1)-xold-dt*((1/2)*(x(1)+xold)-A*(1/2)*(x(1)+xold)*(x(2)+yold));
F(2)=x(2)-yold-dt*(-(1/2)*(x(2)+yold)+B*(1/2)*(x(1)+xold)*(x(2)+yold));
end
but I keep getting the error code
Caused by:
Failure in initial objective function evaluation. FSOLVE cannot continue.
Actually , I was quite confused anyway when I tried to implement it since I was not sure how to deal with the initial values and it seems like i incoorporated them twice..I hope somebody can help me !
Thanks.
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Réponse acceptée
Alan Stevens
le 15 Août 2020
You can do it this way:
A = 0.01;
B = 0.01;
dt = 0.01;
t = 0:dt:20;
x = zeros(size(t));
y = zeros(size(t));
x(1) = 20;
y(1) = 20;
for i = 1:length(t)-1
xnew = x(i); ynew = y(i);
err = 1;
while err > 10^-6
xold = xnew; yold = ynew;
xav = (xnew+x(i))/2;
yav = (ynew+y(i))/2;
xnew = x(i) + dt*(xav - A*xav*yav);
ynew = y(i) + dt*(-yav + B*xav*yav);
err = max(abs(xnew-xold), abs(ynew-yold));
end
x(i+1) = xnew;
y(i+1) = ynew;
end
plot(t,x,t,y)
xlabel('t'),ylabel('x and y')
legend('x','y')
to get:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/346425/image.jpeg)
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