来自《耶鲁大学开放课程：基础物理 第2集 第2集》

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  This is A + A. We're going to call it 2A.
  这就是 A + A 我们也把它称为 2A
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  The beauty of that is now we have discovered a notion of
  现在最美妙的就是我们就已经知道
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  what it means to multiply a vector by a number.
  数字乘以矢量的意义
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  If you multiply it by 2,
  如果拿它乘以2
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  you get a vector two times as long.
  你就得到一个两倍长的矢量
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  Then you're able to generalize that and say,
  然后可以将其推广到更一般的情况
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  if you multiply it by 2.6,
  如果用2.6乘以它
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  I mean a vector 2.6 as long as A.
  所得矢量即为矢量 A 的2.6倍
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  So multiplying a vector by a number
  所以用数字乘以矢量
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  means stretch it by that factor.
  就相当于将它伸长了相应的倍数
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  Then, I want to think of a vector that I can call -A.
  现在我有一个矢量 我将其称为 -A
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  What do I expect of -A?
  我希望 -A 是什么样的呢