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Here I have a system of nonlinear differential equations:

$ (M+2m)\ddot{x} + m(l_1 \ddot{\theta}_1\cos\theta_1 - l_1\dot{\theta}_1^2\sin\theta_1) + m(l_2\ddot{\theta}_2\cos\theta_2-l_2\dot{\theta}_2^2\sin\theta_2) = F $

$ l_1\ddot{\theta}_1 + \ddot{x}\cos\theta_1 - g\sin\theta_1 = 0 $

$ l_2\ddot{\theta}_2 + \ddot{x}\cos\theta_2 - g\sin\theta_2 = 0 $

States are defined by me like this:

$x_1 = \theta_1 , x_2 = \dot{x_1} = \dot{\theta}_1,x_3 = \theta_2, x_4 = \dot{x_3} = \dot{\theta}_2,x_5 = x, x_6 = \dot{x_5} = \dot{x}$

I mean actually what I need to do is to linearize this system about all the states are equal to $0$. But I cannot find the $\cdots$ places below

$ \dot{x_1} = x_2 \\ \dot{x_2} = \cdots \\ \dot{x_3} = x_4 \\ \dot{x_4} = \cdots \\ \dot{x_5} = x_6 \\ \dot{x_6} = \cdots \\ $

How can I find $\dot{x_2},\dot{x_4},\dot{x_6}$?

Thanks

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First you need to solve your system for $\ddot x,\,\ddot \theta_1$ and $\ddot \theta_2$. After that you can find what you are looking for. –  Artem Oct 29 '12 at 18:37
    
I will use Cramer's rule, is this ok for the system of nonlinear differential equations? I mean for $\ddot{x}$... I have done this... I will post it again, will you look at? Thanks by the way. –  kemal acikgoz Oct 29 '12 at 18:42
    
$ \ddot{x} = (F*l_1*l_2 + l_1^2*l_2*m*x_2^2*sin(x_1) + l_1*l_2^2*m*x_4^2*sin(x_3) - g*l_1*l_2*m*cos(x_1)*sin(x_1) - g*l_1*l_2*m*cos(x_3)*sin(x_3))/(M*l_1*l_2 + 2*l_1*l_2*m - l_1*l_2*m*cos(x_1)^2 - l_1*l_2*m*cos(x_3)^2) $ –  kemal acikgoz Oct 29 '12 at 18:45
    
Your system is linear with respect to the second order derivatives. And I do not think the Cramer's rule is the best option in this situation. –  Artem Oct 29 '12 at 18:47
    
ok, what else I can do? I found $\ddot{x}$ by using Cramer's rule as above. –  kemal acikgoz Oct 29 '12 at 18:49
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