A trailer traveling along an uneven straight road can be described by the single-degree-of-freedom model shown on the right. The longitudinal section of the road can be simplified as a sine wave with a wavelength of 1200mm and a height difference of 50mm between the wave peak and the wave trough. The trailer is supported on the axles with springs. If a load of 750N is applied to the trailer, the leaf spring deforms downward by 3mm. The trailer mass is 500kg It is assumed that the trailer is always in contact with the road during driving. Disregarding damping and wheel mass, try: 1) Derive the vibration equation of the system: 2) The trailer is traveling at a constant speed of 25km/h, and the amplitude of the trailer's vertical vibration is: 3) At what speed is the trailer's vertical vibration amplitude the largest; 4) Load 1500kg of cargo in the trailer, and then solve 2) and 3) m