理想状态下的市场能够达到高效生产：将总剩余(total surplus)最大化。 但是在现实中的市场并非完美。在本课程中，我们将探索一系列市场存在着缺陷的情形并且理解为什么会出现这些缺陷；同时，我们会了解一些改进措施，比如反托拉斯(antitrust)政策以及其他一些政府的干预措施。 我们会选取日常中的例子——从我们每天购买和享用的货物和服务开始。我们也会在每周的练习里把理论运用到时下大事和政策研讨中。 所有的这些都会使你成为一个可以理解、分析和评价市场结果的，有学识的，有批判精神的思考者。

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From the course by 宾夕法尼亚大学

微观经济学：当市场“失灵”时

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理想状态下的市场能够达到高效生产：将总剩余(total surplus)最大化。 但是在现实中的市场并非完美。在本课程中，我们将探索一系列市场存在着缺陷的情形并且理解为什么会出现这些缺陷；同时，我们会了解一些改进措施，比如反托拉斯(antitrust)政策以及其他一些政府的干预措施。 我们会选取日常中的例子——从我们每天购买和享用的货物和服务开始。我们也会在每周的练习里把理论运用到时下大事和政策研讨中。 所有的这些都会使你成为一个可以理解、分析和评价市场结果的，有学识的，有批判精神的思考者。

From the lesson

Costs and Profits + Perfect Competition

In the first part of the course we learnt that if we allow market forces to work we reach an efficient outcome: the maximum benefit that can be generated by a market. The second part of the course explores cases where the markets fail to accomplish our goals. This week sets up the benchmark case of the perfectly competitive market: a model we will modify in the next few weeks. We define Perfect Competition, learn to model it graphically and discuss some key results in terms of long run profits and implications for efficiency.

- Rebecca SteinSenior Lecturer

Economics

[MUSIC]

Let's spend just a couple more minutes thinking about these cost curves.

So again, the total cost is comprised of fixed costs and variable costs.

So this average total cost curve

is the average of the fixed costs plus the average of the variable costs.

The average of the fixed cost is always falling

because it's the fixed cost divided by quantity and

the bigger is the quantity, the lower are the average fixed cost.

But this variable cost is more complicated,

because the variable cost increase with quantity, so

we're taking something that's increasing by quantity and dividing it by quantity.

But our marginal cost helps us think about this relationship.

As long as the margin is below the average,

it must be pulling the average down.

As I tell my students, if you get a grade that's below your GPA,

below your grade point average, your grade point average will be dropping.

And once the margin is above the average, it will be pulling the average up.

Again, you get a grade that's higher than your average grade,

your average grade will be increasing.

So let's summarize.

We've looked at two costs, the marginal cost and the average total cost.

We know the general shape of the marginal cost,

the general shape of the average total cost and the relationship between the two.

When the margin is below the average, it pulls the average down.

When the margin is above the average, it pulls the average up.

I think we're ready to think about Firm's profit maximization.

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