[填空题]
The predominant frequency xsg 1zt.(w.l uof a certain fire eoj 6td/7; zewj ozpl,.ngine’s siren is 1550 Hz when at rest. What frequency do you detec7ozjolp.6t; j/ ,wez dt if you move with a speed of 30 m/s
(a) toward the fire engine, Hz
(b) away from it? Hz