“一人在一舟中距岸上之最近点为2哩,今欲以最短时间达到岸上离最近点6哩处,设该人每小时能划行4哩,步行5哩,则应向岸上何点划行?”——这是范氏大代数中习题43中的第10题。如图所示,设PQ代表河岸,河中一舟在A点,它距岸上最近之点为B,则AB=2哩。点C到点B处之距离为6哩,现在假定小舟向D点划行,而BD=x哩,则DC=(6-x)哩。这样该人由A点经D点而到达C点之时间t为: “The nearest point in a boat to the shore is 2 miles. I would like to reach the 6-foot distance from the nearest point on the shore in the shortest time. Let this person be able to row 4 miles per hour and walk 5 miles to the shore. Rowing?”—This is the 10th problem in exercise 43 in the Fantasy algebra. As shown in the figure, let PQ be the bank of the river and a boat in the river at point A. The nearest point on the bank is B, then AB=2. The distance from point C to point B is 6 inches, and now it is assumed that the boat drew to point D, and BD=x哩, then DC=(6-x)哩. In this way, the time t from point A to point C in point A is: