Two loudspeakers are placed side by side a distance d apart. A listener observes constructive interference while standing in front of the loudspeakers, equidistant from both of them. The distance from the listener to the point half-way between the speakers is l. One of the loudspeakers is then moved directly away from the other. Once the speaker is moved a distance r from its original position, the listener, who is not moving, observes destructive interference for the first time. Find the speed of sound v in the air if both speakers emit a tone of the same frequency f. http://session.masteringphysics.com/problemAsset/1000054/11/104507C.jpg ______________________________ I know that velocity = (wavelength)(frequency), and the path length difference for the case of destructive interference is =0.5(wavelength). And the distance between the observer and the speaker that has been moved is sqrt((0.5d+r)^2 + l^2). How do I put everything together to get the speed of sound in the air using only those variables introduced in the problem????