来自《耶鲁大学开放课程：基础物理 第13集 第13集》

• 
  
  Let me divide this by this on the left-hand side to get w.
  在左边用这个除这个得到 w
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  When I write Δx' over Δt',
  当我写 Δx' 除以 Δt'
• 
  
  you guys take the limits of everything going to 0.
  大家取所有趋于零的极限值
• 
  
  I don't feel like writing that.
  我不喜欢写那个
• 
  
  In fact, it's true even without the limit,
  事实上 不取极限也是对的
• 
  
  but let's apply it in the end to an instantaneous velocity.
  最后让我们把它应用于瞬时速度中
• 
  
  What happens on the right-hand side?
  右边会怎么样
• 
  
  Here is what Δx' is equal to.
  Δx' 就等于这个
• 
  
  Can you do this in your head?
  你能心算出来吗
• 
  
  If this Δx-u·Δt,
  如果这是 Δx-u·Δt
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  this denominator cancels between dividing this by this,
  用这个除以这个 约去分母
• 
  
  so it looks like--
  就是这样的