来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  For an ellipse,
  要画一个椭圆
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  you take two thumbtacks and you take a string like this,
  你拿两个图钉和一根弦 像这样
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  keeping that length plus that length constant,
  保持这两个长度的和一定
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  and you move, keeping the string tight,
  然后你移动画笔 保持弦紧绷
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  and the shape you produce is the ellipse.
  你得到的图形就是椭圆
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  And these are the two focal points, F1 and F2.
  这是它的两个焦点 F1 和 F2
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  An ellipse is an egg-shaped circle
  椭圆是一个蛋状的圆
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  and if you want to get the circle back from the ellipse,
  如果你想要把椭圆变回圆
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  you just move the two focal points to a single point,
  你只要移动两个焦点到一个点上
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  and you'll get the circle.
  你就可以得到一个圆
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  Whereas a circle has only a single number R,
  圆只有一个参量 R
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  the radius characterizing it,
  也就是半径