来自《耶鲁大学开放课程：基础物理 第6集 第6集》

• 
  
  you notice you don't do anything to y;
  你会发现和 y 没有关系
• 
  
  y will be left fixed at whatever y you had;
  不管 y 是什么 它的值都不会变
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  x will be changed by Δx, you find the change;
  x 会变化 Δx 求出这个变化值
• 
  
  you take the derivative and take the limit
  求导数然后求极限
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  that is denoted by the symbol df / dx.
  这用符号 df / dx 表示
• 
  
  So, this curly d instead of the straight d
  这个弯曲的 d 而不是直的 d
• 
  
  tells you it's called a "Partial derivative."
  说明这是偏微分
• 
  
  Some people may want to make it very explicit
  有人可能想表示得清楚一些
• 
  
  by saying this is the derivative with subscript y.
  这可以通过下标 y 说明
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  That means y is being held constant when x is varied,
  说明在 x 变化的时候 y 保持不变
• 
  
  but we don't have to write that
  但我们不必写
• 
  
  because we know we've got two coordinates.
  因为我们只有两个坐标