[填空题]
In one of the original Doppler u bs gx /h78z/yj7ei3wn)lv0j experiments, a tuba was played on o3r ib4yip. nr43 9t g+icz/ufwru. 6ya 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 traoi+fp4uu33ry9wi n4 y gib. 6/c.rzrtin car approached the station at a speed of 10 m/s ? Hz
