来自《耶鲁大学开放课程：基础物理 第10集 第10集》

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  But in this problem, there's a very nice solution.
  但在这里 有一个非常好的解决办法
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  There's stuff I don't want that is going to vanish.
  我不想要的项会自动消失掉
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  It's going to vanish.
  它就会消失掉了
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  And let's see why it vanishes.
  来找找看它为什么消失
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  Let's take the cross terms.
  先找到交叉项
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  The cross terms are like the 2A dot B terms.
  交叉项就像 2A 点乘 B
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  So, here is the 2.
  所以这里是系数 2
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  One of the vectors A is the vector d.
  其中一个向量 A 是 向量 d
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  And it's going to be taking the dot product with mi·ri summed over i
  这里是点乘 mi·ri 对 i 的求和
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  because there's going to be a d dot ri twice.
  因为这里有两倍的 d 点乘 ri
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  But d is not dependent on the summation index.
  但是 d 与求和指标无关
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  It's common to all the terms in the sum and you pull it out.
  它对所有项来说都是常量 可以提出来