20. Two sources are 5.4 cm apart and vibrate in phase at 6.0 Hz. A point on the third nodal line is 20.0 cm from one source and 26 cm from the other. a) CaICulate the wavelength of the waves.[2 marks] b) Calculate the speed of the waves. [2 marks] 21. Two towers of a radio station are 0.6 km 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 108 mfs. Determine the rst (non-zero) angle at which the radio signal strength is at a maximum for listeners who are on a line 200 km north of the station (c : 3 x 108 m/s). [4 marks]