1. The problem statement, all variables and given/known data Hello, My first post here I have a numerical problem for Matlab but I get stuck with the basic math... For a circuit I have three equations: 1.Inductance: L=Lo/(1+I^2) 2.Voltage over the inductance: V=L*dI/dt 3.Current over a condensator: I=-C*dV/dt 2. Relevant equations From this I should be able to derive the second order differential equation: 4. d2/dt2=2I(dI/dt)^2/(1+I^2)-I(1+I^2)/Lo*C I know I must find the derivative for V, dV/dt and substitute this into I=-C*dV/dt and this should yield equation 4 but I think my derivatives are all wrong! 3. The attempt at a solution Basically I need help to use the chain rule (i guess?) and finding the derivative for V=L*dI/dt This is what I got: dV/dt=dI/dt*(-Lo*2I)/(1+I^2)^2+L*d2/dt2*??? something is wrong... Would really appreciate the help!