来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  The mass of the shaded region is the mass per unit area.
  每个小窄条的质量用单位面积的质量表示
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  I'll find the area later on in terms of b and h,
  我会通过 b 和 h 来计算面积
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  but this is mass per unit area.
  但这里的质量是单位面积上的质量
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  Then, I need to know the area of the strip.
  然后 我需要知道小窄条的面积
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  I'm going to give the answer
  我会直接给出答案
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  because I don't have time to probe it.
  因为没时间来进行求解
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  But you should think about
  但你们应该思考一下
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  what the answer for the area of the shaded region is.
  该如何计算图中阴影部分的面积
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  It's got a height 2·y, and it's got the width dx.
  它的高是 2·y 宽是 dx
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  It's not quite a rectangle
  它并不是矩形
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  because the edges are slightly tapered,
  因为有两条边还是微微倾斜的
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  but when δx goes to 0,
  但当 δx 趋于 0 时