MATLAB: Represent differencital equation with ode45

ode45

i got this differential equation:
function xdot=tresorden(t,x)
xdot=zeros(3,1);
Vp=5;
Vi=Vp*square(2*pi*t)+5;
xdot(1)=x(2);
xdot(2)=x(3);
xdot(3)=6*Vi-6*x(1)-11*x(2)-6*x(3);
xdot=[xdot(1);xdot(2);xdot(3)];
how can i represent x(1)?

Best Answer

i'm going to collect what i have so far. this is the function:
function xdot=tresorden(t,x)
xdot=zeros(3,1);
Vp=5;
Vi=Vp*square(2*pi*t)+5
xdot = [x(2, :);
        x(3, :);
        6 * Vi - 6 * x(1, :) - 11 * x(2, :) - 6 * x(3, :)];
those are the commands:
%to evaluate the function
[t,x]=ode45('tresorden',[0,10],[0,0,0])
%to plot the x''
plot(t, x(:, 2));
hold on
%to plot the x'''
xdot   = tresorden(t,x.').';
plot(t, xdot(:, 3));
%this should plot the x''' but it doesn't, as you can check in the graphics
in the graphics you can see that in the matlab plot the blue graphic (x''') is not the derivative of the red graphic (x'')
in the simulink output you can see that the blue graphic (x''') IS the derivative of the red graphic (x'')
i can't uderstand what is happening, perhaps there is a mistake on the definition of the function in matlab.
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