[填空题]
The predominant frequen1kwaibhc 8hx7o+6y4n.7 4ow d ini9rncy of a certainky. sf0tq) w04ocbfbz;6 f y ;kp2xrwmuv ./m( fire engine’s siren is 1550 Hz when at rest. What frequency do you detect if you move with a spey4k0f(pbub/0t )2mkq of;;sfxrmw. vywzc .6 ed of 30 m/s
(a) toward the fire engine, Hz
(b) away from it? Hz
