①y1=60×5×S÷50+60×2×S+3000=126×S+3000 y2=60×5×S÷80+60×1.7×S+4520=105.75×S+4620
②当y1=y2时, 解得s=80(km)
所以当s>80 km时,选择火车合算 当s