[填空题]
Two automobiles are equipped wid:3o l/wemzu0 n.4y arth the same single frequency horn. When onelida+9,)k*eypf8 zn;es cwn 2gm/0 k e is at rest and the e)2;skc/em+ nnwla ie8 *kg p9z,0fy dother is moving toward the first at 15 m/s the driver at rest hears a beat frequency of 5.5 Hz. What is the frequency the horns emit? Assume T = 20 $^{\circ} $C Hz