来自《耶鲁大学开放课程：基础物理 第10集 第10集》

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  Okay, it's like putting a washing machine or something,
  就像安装洗衣机一样
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  and when you're done you find three or four parts on the ground.
  当拼完时你发现还有三四个零件在地上
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  You don't know why they're there.
  你不知道这是为什么
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  In that case, there's no solution to that problem.
  那样的话 这个问题就没有得到解决
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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