[填空题]
The predominant frequency of a cespx,0o,9u+y . qxmivq9ez4ev rtain fire engine’s sirent 5kt/ 9f qk9pw-/5kc/bki zqh is 1550 Hz when at rest. What frequency do you detect if you move tpkqb9ktkczq9w5 -/ / / hkif5with a speed of 30 m/s
(a) toward the fire engine, Hz
(b) away from it? Hz