The math problem about the shortest distance is urgent! ! !
Let point A be the symmetrical point A' about the river, and point P connect A'B and the river, then point P is the shortest distance where the bridge is built, and PA+PB=A'B, which helps you prove that if there is any point P' on the river, then the sum of the distances from P' to A and B is A'P'+BP', which is longer than A'B and is based on a triangle.