来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  in the limit in which they are approaching 0,
  当它们趋近于零时
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  the two will be, in fact, equal.
  式子两边就完全相等了
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  If Δx is a finite amount, like 1 second,
  如果Δx是一个有限大小的量 例如1秒[此处应为老师口误]
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  this will not be true because in the starting equation,
  这式子就不对了 因为在原始方程里
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  Δx and Δt and Δ(v^(2)) were all assumed to be infinitesimal.
  Δx Δt以及Δ(v^(2))都假设为无限小
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  So don't hesitate to do manipulations of this type,
  所以不要犹豫 直接就这么做
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  and I will do them quite often.
  我经常这么做
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  So you've got to understand
  你必须得知道
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  when it's okay and when it's not okay.
  什么时候可以 什么时候不可以
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  What this means is, in a time Δt,
  也就是说 在Δt时间内
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  this quantity changes by some amount,
  这个变量有一定变化
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  and in the same time, Δt,
  在同样的一段时间Δt内