Resonance-driven oscillator

The equation y'' + y = cos(x) models an oscillator forced exactly at its natural frequency (delta < 0 for the homogeneous case, with p = 0 and q = 1). Explain why a particular solution of the form y_p = Acos(x) + Bsin(x) cannot be used here, suggest the correct form to try, and then determine the corresponding particular solution. Finally, comment on the behaviour of this solution as x tends to infinity.