来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  times cosine of the angle;
  乘以它们夹角的余弦
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  add them all up.
  最后全部相加
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  Then somebody will say,
  这时会有人说
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  "Well, your segments are not short enough for me."
  "你的小片段对我来说还不够小"
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  We'll chop it up some more, then chop it up some more,
  我们可以继续分割 再分割
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  chop it up till your worst critic has been silenced.
  分割到你满意为止
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  That's the limit
  这里的极限
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  in which you can write the answer as this integral.
  可以用积分形式表示出结果
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  Just like in calculus when you integrate a function
  就像在微积分中求积分
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  you take tiny intervals Δx, multiply F by Δx,
  取微元 Δx 用 Δx 乘以 F
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  but then you make the intervals more and more numerous
  分割的越来越多
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  but less and less wide
  长度就越来越小