[填空题]
A factory whistle emv(, 6h4cc vga*y(vfk gi 7y( hr+jtxkl0ts sound of frequency 570 Hz. When the wind velocity is 12 m/s from the north, what frequency wil0lhjkt(x+y 7rl 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