The meaning of this question is: there is a square pond with a side length of ten feet. There is a reed in the center of the pond, and the reed is one foot above the water. If you pull the reed to the shore, it will only touch the shore. What is the depth of water and the height of reeds? This is a very old problem, and today we might as well use Pythagorean theorem to solve it. As shown in the figure, let the water depth BD be x feet, because CD= 1 foot, so there is AB=BC=x+ 1 (foot) and the side length of the pond is ten feet, so AD is half of the side length, that is, AD=5 feet. According to Pythagorean theorem, there are: AD 2+BD 2 = AB 2, that is:

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