来自《耶鲁大学开放课程：金融市场 第2集 第2集》

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  [P(x=xi) times (xi)].
  求和
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  I have it down that there might be an infinite number
  对于这个随机变量X
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  of possible values for the random variable x.
  可能的取值个数是无限的
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  In the case of the coin toss, there are only two,
  在抛硬币的例子里 只有两个取值
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  but I'm saying in general there could be an infinite number.
  但是一般都会有无限个可能值
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  But they're accountable and we can list all possible values
  但随机变量是离散的话
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  when they're discrete and form a probability weighted average
  我们可以把所有的可能值列出来
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  of the outcomes.
  然后算出加权平均值
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  That's called the expected value.
  这个加权平均值就是期望值
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  People also call that the mean or the average.
  也可以叫做平均值
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  But, you know that this is based on theory.
  但注意 这个公式是基于概率论的
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  These are probabilities.
  这些数值是概率