18. Two towers of a radio station are 400 m apart along an east-west line. The towers act as point sources radiating at a frequency of 1.0 x 106 Hz. Radio waves travel at a speed of 3.0 x 105m/s. Determine the first angle at which the radio signal strength is at a maximum for listerners who are on a line 20.0 km north of the station (c = 3 x 108 m/s). [4 marks]