来自《耶鲁大学开放课程：金融理论 第2集 第2集》

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  So here xi is bigger than eix.
  这是xi比eix大
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  You're buying x.
  你就买x
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  Here yi is less than eiy.
  这yi比eiy小
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  You're selling Y.
  你就卖出y
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  And so the revenue you get from selling Y
  卖出Y的收益
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  equals the expenditure you make on buying X.
  等于购买X的花费
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  So the cleverness is in realizing it doesn't matter what the signs are.
  聪明之处在于不管这些指标是多少
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  If xi is less than eix
  如果xi比eix小
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  this equation still makes sense
  这个等式依然成立
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  because then you get a negative number.
  因为你得了一个负数
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  You've gotten money by consuming less X than you started with
  你通过消费比开始少的x来获得资金
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  so that's money you can use to buy Y.
  剩下的钱能用来买y