来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  The change in the function is a change in the function.
  函数的变化量就是这么多
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  You've got to get the same answer both ways.
  通过两种方法得出的结果是一样的
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  That's the reason;
  这就是原因
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  that's the requirement that leads to this requirement.
  也就是它成立的前提条件
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  Now, I will not be keeping track of functions
  对于今天会讲到的函数
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  to this accuracy in everything we do today.
  我就不会再这么详细地分析了
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  We'll be keeping the leading powers in Δx and Δy.
  只考虑包含 Δx 和 Δy 的低阶项就行
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  So, you should bear in mind
  请大家牢记
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  that if you make a movement in the xy plane,
  如果物体在 xy 平面内移动
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  which is Δx horizontally and Δy vertically,
  即 Δx 表示水平位移而 Δy 表示竖直位移
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  then the change in the function is this .
  那么这就是函数的变化量
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  Draw a box around that, because that's going to be...
  我把它框起来了 因为这会是