A source of sound with natural frequency vo = 1.8 kHz moves uniformly along a straight line separated from a stationary observer by a distance l = 250 m. The velocity of the source is equal to η = 0.80 fraction of the velocity of sound. Find:
(a) The frequency of sound received by the observer at the moment when the source gets closest to him;
(b) The distance between the source and the observer at the moment when the observer receives a frequency v = vo.