[填空题]
In one of the original Doppler experiments, a tuba q4lq xdgi,79xwas p w()oqkg tl:r1layed on a moving flat train car at a frequency of 75 Hz, and a second identical tuba played the same tone while at rest in the railway station. What beat frequency was heard if the train ca k):(g1wqlrtor approached the station at a speed of 10 m/s ? Hz
