[填空题]
A factory whistle emits sound of frequency 5 re3safxspdp-d7,v, trf+ )8k70 Hz. When the wind velocpdw :*schzn f6/k/a,kity is 12 m/s from the north, what fr,k/nzf:* p h kscw/d6aequency will observers hear who are located, at rest,
(a) due north, Hz
(b) due south, Hz
(c) due east, Hz
(d) due west, of the whistle? What frequency is heard by a cyclist heading Hz
(e) north Hz
(f) west, toward the whistle at 15 m/s? Hz
Assume T = 20 $^{\circ} $C
