[填空题]
Two loudspeakers are 1.80 m /dx g/i7ni9le apart. A person stands 3.2 qnrun-buy8: 00 m from one speaker and 3.50n -b 8y2unur:q m from the other.
(a) What is the lowest frequency at which destructive interference will occur at this point? f = Hz
(b) Calculate two other frequencies that also result in destructive interference at this point (give the next two highest). Let T = 20 $^{\circ} $C $\approx$ Hz $\approx$ Hz
