Does that integral trick to find the unknown U_i functions in variation of parameters work for linear diff eqs of order higher than 2? My professor told me that it only works for 2nd order problems but I did a problem on Paul's online notes that was 3rd order problem and he used those integrals to solve for the unknown functions. And by the way when I say he used the integral trick I know you still have to integrate the derivative of the unknown function to get the function itself but I mean having the trick where you dont have to do any linear algebra to do variation of parameters.