来自《耶鲁大学开放课程：博弈论 第9集 第9集》

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  It must be the case that both l is a best response and r is a best response
  策略l和r一定属于最佳对策
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  in which case the payoff must be,
  也就是说它们的收益是怎样的
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  someone shout it out, equal, thank you.
  谁来说一下 相等 很好
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  They must be equal, these must be equal.
  它们的收益必然相等
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  They must be equal since Serena is indifferent between
  因为塞丽娜选左和选右是一样的
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  choosing left or right and hence is mixing over them.
  所以它们二者必然相等
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  So again, using the fact that they're equal reduces this to algebra,
  利用上面的条件同样又约掉了一个未知数
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  and again, I'll probably get this wrong but let me try.
  我可能会算错 大家注意一下
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  So I claim, let's take 20 away from here,
  从这里减去20之后
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  I've got 30P equals 70(1-P).
  可以得到30P=70(1-P)
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  I hope that's right, that looks right.
  我应该没有算错
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  Again, this is just algebra at this point.
  重复一下 我们只用到了代数知识