First calculate the wavelength: u/f=2m. Taking A as the origin of coordinates and the coordinate of a point on a straight line as X, the phase difference of two vibrations at X consists of two parts: the phase difference of AB itself and the phase difference caused by different distances to AB. The phase difference of AB is π. When the phase difference of vibration generated by AB at X is [x/2-(20-x)/2] * 2π+π = (2k+1) π, it is steady, and the solution is X = 10+K. K from -9 to 9.