@ Nonlinear ode equations

Hi!

I am working on nonlinear ODE equations -

M2*X2" + C2*(X'2-X'1) + K2*(X2-X1) = 0

M1*x1" - C2*(X'2-X'1) - K2*(X2-X1) + K1*(X1-Xr)-Knl*(X1-Xr)^2= 0

Plz, Can anybody help me in solving these nonlinear ODE?

Is there any method to obtain the state space model of this system?

Mac.