[填空题]
In one of the original Dopplmm0lv4 i-kt8 wm2g-sper e4ja)yz: -os) f +b 1mcz k2hr 6bmpojfsc)7pb2xperiments, a tuba was played on a moving flat train car at a frequency of 75 Hz,y 2-:7+mmbba j bpzo)f)6pjkrfsz cco1s) 2 4h and a second identical tuba played the same tone while at rest in the railway station. What beat frequency was heard if the train car approached the station at a speed of 10 m/s ? Hz
