[填空题]
The frequency of a steam train whistle as it approaches -i/cmn s+1xqm 2b;dko youxw4ut.+z4im 3 h t j26wj1dfxp is 538 Hz. After it passes you, its frequency is measured as 4862mdwwz4p6. xfj 4+h3ijx u tt1 Hz. How fast was the train moving (assume constant velocity)? m/s
