For the second order DE in number 5, when we had to find C and D etc. Finding C was trivial (t=0, x=0), but when you differentiate to find D, when t=0 does dx/dt=0? I had that originally, but if B is moving at 1/4sin2t, isn't dx/dt 1/2cos2t, ie 1/2? 0 or 1/2?