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The other day a student came to ask me for help in solving a second order ordinary differential equation using the ode45 routine of MATLAB.  To use ode45, one needs to be familiar with how the inputs are required by MATLAB.  The understanding of these inputs is important to use ode45 successfully in problems that are more complex than solving a second order ODE.

The ordinary differential equation was
      2y”+3y’+5y=7 exp(-x), y(0)=11, dy/dx(0)=13
This has to put in the state variable form by reducing it by using
      y’=z
That gives
      y’=z with the corresponding initial conditions as y(0)=11
Then
      2y”+3y’+5y=7 exp(-x)
reduces to
      2z’ + 3z+5y=7exp(-x)
      z’ =(7exp(-x)-3z-5y)/2 with the corresponding initial conditions as z(0)=13

So as needed by MATLAB, call y as y(1) and z as y(2)
       dy(1)=y(2), y(1) at x=0 is 11
       dy(2)=(7exp(-x)-3y(2)-5y(1))/2, y(2) at x=0 is 13

These equations are now put in a MATLAB function we call odestate.m
       dy=zeros(2,1);
       dy(1)=y(2);
       dy(2)=(7*exp(-x)-3*y(2)-5*y(1))/2;

To solve the ODE, the
The inputs are
1) the function odestate
2) The outputs are required between x=0 and x=17,
     hence entered as [0 17]
3) The initial conditions are y(0)=11 and dy/dx(0)=13,
     hence entered as [11  13]

The outputs are
1) X= array of x values between 0 and 17
2) Y= matrix of 2 columns;
      first column is the y(x)
      second column is dy/dx(x)
The MATLAB code then is
[X,Y]=ode45(@odestate,[0  17],[11 13]);

Click the links for the MATLAB mfiles for the function odestate.m and the ODE solver odetest.m

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This post is brought to you by Holistic Numerical Methods: Numerical Methods for the STEM undergraduate at http://numericalmethods.eng.usf.edu, the textbook on Numerical Methods with Applications available from the lulu storefront, and the YouTube video lectures available at http://numericalmethods.eng.usf.edu/videos and http://www.youtube.com/numericalmethodsguy

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