The drawing shows a loudspeaker a and point c, where a listener is positioned. a second loudspeaker b is located somewhere to the right of a. both speakers vibrate in phase and are playing a 68.6-hz tone. the speed of sound is 343 m/s. what is the closest to speaker a that speaker b can be located, so that the listener hears no sound? (hint: the law of cosines may be here.)