来自《耶鲁大学开放课程：金融理论 第4集 第4集》

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  It means that setting derivative equal to zero implies
  重点是导数等于零意味着
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  you are at max.
  你达到了最大化
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  So that's not true of any function.
  这和任何函数的概念都是矛盾的
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  If you have a function like that and then it goes up like
  如果有一个函数像这样 并如此上升
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  that you could have the derivative of zero here and not
  你可能会得到导数为零 但不一定最大化
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  really be at the max, but because there's diminishing
  因为其减小的X的边际效用会月俩月大
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  marginal utility more and more of X does less and less good
  其效用也会越来越不如原来的好
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  so it turns down.
  然后向下转折
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  So all you have to do is figure out that the derivative is zero,
  你能做的就是找出哪里的导数为零
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  which it is at this point, and then you know you're at the maximum.
  因为在导数为零的点上你知道你处于最大化
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  But that at X^ = 77 has to be the case because if you plug
  但是X^a=77必须成立 因为
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  in X^ = 77 and you take the derivative of all this with
  如果你使X^a=77 且与X^a相关的