This is the answer to the question 13 on page 95 of the eighth grade mathematics book of Beijing Normal University Press: (1) The line segment between two points is the shortest, (2) The distance between the vertical line and the two ends of the line segment is equal, and the problem can be solved by using the above principles. Solution: If the bridge is CD, then the route in this problem is the sum of AC, CD and DB. How to convert between two points into one? After observation, it is not difficult to find that the line segment CD is a constant value, and we only need to consider making AC+DB the shortest. They are two scattered line segments. Let's translate one of them first, as shown in the figure, from DB to CB'. At this time, we connect AB' and cross L at P to get the bridge address. (1) Translate point A vertically downward, and the translation distance is equal to the street width, reaching point A 1. Connect A 1B, cross the street edge near b at B 1, and build a bridge through B 1. (2) Make point B a symmetrical point B2 about the street, connect AB2, make it the middle vertical line of AB2, intersect with the edge of the street near A at point A2, and build a bridge through point A2 will meet the requirements.