来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  find the integral of the work done by the force
  求出力从 r1 到 r2
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  between r1 and r2,
  所做功的积分
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  you will have to tell me--
  你们会说
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  you'll have to ask me something. You agree?
  你们会问我些什么 同意吗
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  What will you ask in addition,
  你们会问什么补充条件
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  besides just saying start at r1 and end at r2?
  除了知道从 r1 到 r2 之外
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  You will ask, "What path did you take?"
  你们会问 "你选择哪条路径"
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  And unless you specify the path you cannot do the integral,
  除非给定路径否则无法求积分
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  because you don't know what contour you want to draw.
  因为你不知道路线
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  Therefore, the answer in general depends on the path,
  因此 答案一般都和路径有关
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  but this right-hand side does not.
  但等式右边和路径无关
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  So, if you pick the same two endpoints
  所以 如果选择两个相同的终点