The core idea of this topic is unchanged, that is, between two points, the line segment is the shortest. After encountering such a problem, you can try to think positively. So first of all, you have to understand that MN is perpendicular to the river bank. Then MN in AMNB=AM+MN+NB is a constant value, that is, how can we "draw" AM+NB into a straight line? 1, about taking the "center line" of the river as the symmetry point B', there is nb = MB'; (The exact location of point M will be determined later) 2. Regarding the riverbank line where point M is located, make point B "the symmetry point of b', so that MB' = MB "；; 3. The problem that the intersection of the riverbank line connecting A and B and M is M has been solved. Hehe ~ think about it yourself and sum it up.