Between two points, the line segment is the shortest, that is, the connecting line between two points is the shortest. There is a bridge between AB in the picture, which is perpendicular to the river, so the length of the bridge is fixed. At the same time, the direction of the bridge is fixed. If AG is perpendicular to the river bank, the length of FG is equal to the length of the bridge. Translating the position of a to e along the direction of AG, AE=FG is equivalent to removing the bridge between AB, and the shortest distance is be at this time.