1, cross a as the vertical line of the river, and intercept AC= the width of the river on the vertical line. 2. Connect CB with the other side of the river and meet at point D (the side near B). 3. crossing d is the vertical line of the river, crossing the other siDE at point e, then de is the bridge! Because the width of the bridge is constant, to make the distance shortest, we must make the distance from A to B shortest. From the picture we can know that CD and AE are parallel and equal! So from A-E-D-B is the shortest path!