Is this DE nonlinear???? 1. The problem statement, all variables and given/known data Determine the order and if this equation is Linear or Nonlinear. 3(d4x/dt4) + 4(dx/dt) + 9x = 3cos(3t) The d4x is a forth order derivative. Sorry for the messiness. Ok, this is a 4th order DE. I think it is nonlinear because of the two products (multiplying 4 time the derivative and 4 times the other derivative). Also, the 3cos(3t) has no bearing on this because t is independent right? If the t was a y, that would cause nonlinearness. Am I right on all this?