来自《耶鲁大学开放课程：基础物理 第2集 第2集》

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  This is a matter of simple algebra,
  本质上讲 这是简单的代数运算
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  of taking this and putting it here.
  把这个式子代入上面的式子
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  But I showed you in the end
  我上节课最后教过你们
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  how we can use calculus to derive that.
  如何用微积分来得到那个结果
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  I got the feeling, when I was talking to some of you guys,
  当我和你们中某些人聊天时 我感觉到
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  that maybe you should brush up on your calculus.
  你们应该好好复习微积分
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  You have done it before,
  你们以前学过
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  but when you say, "I know calculus, "
  但当你们说 "我懂微积分"
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  Sometimes it means you know it,
  有时这表明你们懂
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  sometimes it means you know of some fellow who does
  有时表示你知道谁懂
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  or you met somebody who knows.
  或者你遇到了懂这个的人
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  That's not good enough.
  那样并不好