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## Main Question or Discussion Point

**1. 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 dont 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.