这个动画，外圆半径为1,内圆为3
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作者:Dave Van Leeuwen

主题:圆

This was created in reply to a question from LinkedIn, asking as to the number of rotations of a circle of radius 1 as it rolls around a circle of radius 4. In this case the point that starts out at the bottom of the small circle is indicated each time it returns to the bottom of that circle. The red path can be graphed on any graphic calculator provided correct parametric equations can be identified.

a circle of radius 1 as it rolls around a circle of radius 4
这句话有歧义，如果 a circle of radius 4指的是指以外圆圆心到内圆的圆心为半径，外圆圆心形成的圆的话这视频就是对的，如果是指内圆的半径就有问题了

从红色箭头指向的外圆圈上点的移动轨迹来看，应该是指外圆圆心到内圆圆心距离为4,因为四个弧度(每个弧度为外圆一周)加起来肯定是大于固定圆的周长的。

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