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**1. Homework Statement**

Two antennas located at points A and B are broadcasting radio waves of frequency 96.0 MHz, perfectly in phase with each other. The two antennas are separated by a distance d=12.40m. An observer, P, is located on the x axis, a distance x=55.0m from antenna A, so that APB forms a right triangle with PB as hypotenuse.

If observer P starts walking until he reaches antenna A, at how many places along the x axis will he detect minima in the radio signal, due to destructive interference?

**2. Homework Equations**

Destructive interference = lambda / 2

lambda = c/f

**3. The Attempt at a Solution**

Since lambda is 3.125 m (3E8 / 96MHz), I thought the total number of times the observer would experience destructive interference would be once every wavelength (where it's lambda / 2) so I took 55 / 3.125 and got 17.6, which would produce 18 destructive interferences. This is incorrect, not sure what I'm doing wrong. Picture is attached. Thanks!