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From: John on 24 Oct 2009 21:21
Hi, I have two coupled 2nd order D.E.'s that I need to put into ode45 and plot. I am having a hard time putting them in state variables. Please help.

X1"=-.75*X1-.75*X2
X2"=-3*X1-.5*X2

Thanks.
From: Torsten Hennig on 25 Oct 2009 23:37
> Hi, I have two coupled 2nd order D.E.'s that I need
> to put into ode45 and plot. I am having a hard time
> putting them in state variables. Please help.
>
> X1"=-.75*X1-.75*X2
> X2"=-3*X1-.5*X2
>
> Thanks.

Set x1' = y1, x2' = y2.
Then your 2nd-order-system becomes a 1st-order system that can be solved by directly applying ODE45:
x1' = y1
y1' = -0.75*x1 - 0.75*x2
x2' = y2
y2' = -3*x1 - 0.5*x2

Best wishes
Torsten.
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