Matlalb problem with two first order ODEs

[sara_87]

Homework Statement

I have two questions:
1) If i have two first order ODE y(1) and y(2) (in terms of time), i know how to plot y(1) versus time and y(2) versus time but i don't know how to plot y(2) versus y(1)

2)I have two second order ODES X''=... and Z''=... to solve this, we make the substitutions y(1)=X, y(2)=X', y(3)=Z, and y(4)=Z'
then we have two first order ODEs: y(2)'=... and y(4)'=...
I typed this into Matlab (dydt=[(the equation for y(2)'; the equation for y(4)']) but an error came up since i didnt define y(4). how do i do this?
2. Homework Equations



3. The Attempt at a Solution
1) i tried putting:
plot(y(1),y(:,2)), and i also tried plot(y(1),y(2)) but these didnt work.


Any help would be very much appreciated.
Than you.

 

[Mindscrape]

1) plot(y(:,1),y(:,2)) ought to work, assuming you just left out the iterate through all rows part in the first column.

2) Can you post your exact system of equations function?