来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  If you've got a function f(x),
  如果有一个函数 f(x)
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  you know the value at some point,
  你知道其在某一点的值
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  and you want to go to a neighboring point,
  然后移动到附近的某个点
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  a distance Δx away.
  比如离开 Δx
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  You ask, "How much does the function change?"
  如果问 "函数变化了多少呢"
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  And the answer is,
  答案是
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  the change in the function Δf
  函数的变化量 Δf
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  is the derivative of the function at the starting point
  等于函数在起点处的导数
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  times the distance you move.
  乘以移动的距离