There is a trick to this kind of problem, as shown in the figure: make point A', the symmetrical point of point A relative to the X axis, and then PA'=PA, and the sum of the distances from p to AB =PA+PB=PA'+PB. Then when P is on the line of A'B, PA'+PB is the shortest distance. Compared with another point p', it is obvious that the shortest distance between two points is used here.