Using circle vector function and medium rack as tools, the equation of contact trace is deduced, and its characteristic is discussed.

以圆矢量函数和媒介齿条为工具，推导了接触迹方程，讨论了接触迹的特性。

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Back then, there had been a big tractor tire hung from a rack to mark the circle (back in Eagle Plains, I had seen the photos of the tire).

当时，出现了大拖拉机轮胎悬挂在机架上，标志圆(在鹰平原，我看到了照片的轮胎)。

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