In Figure a microwave transmitter at height a above the water level of a wide lake transmits microwaves of wavelength ? toward a receiver on the opposite shore, a distance x above the water level. The microwaves reflecting from the water interfere with the microwaves arriving directly from the transmitter. Assuming that the lake width D is much greater than a and x, and that ? > a, find an expression that gives the values of x for which the signal at the receiver is maximum.