来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  Now, you've got to realize
  到现在你们必须要知道
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  that when you're doing these calculus problems,
  在解决微积分的问题时
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  Δx is a tiny number, Δy is a tiny number,
  Δx 是个很小的数 Δy 是个很小的数
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  Δx·Δy is tiny times tiny.
  Δx·Δy是小数乘以小数
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  So normally, we don't care about it,
  所以一般我们不管它
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  or if you want to be more accurate, of course,
  当然 如果你想更精确一些的话
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  you should keep that term.
  你应该保留这一项
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  In the first approximation,
  对于一级近似
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  where you work to the first power of everything,
  当解决的都是一次项时
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  this will be your Δf.
  这就是要求的 Δf
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  But if you're a little more ambitious
  如果你更有追求的话
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  but you keep track of the fact the derivative itself is changing,
  你留意到导数本身也在变化