请教平狄克的一道课后复习题

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原题为：一个厂商的短期供给曲线是短期边际成本曲线（位于最小平均可变成本的点之上），为什么长期供给曲线不是长期边际成本曲线（位于最小平均成本的点之上）？

所给的答案是：

In the short run, a change in the market price induces the profit-maximizing firm to change its optimal level of output. This optimal output occurs when price is equal to marginal cost, as long as marginal cost exceeds average variable cost. Therefore, the supply curve of the firm is its marginal cost curve, above average variable cost. (When the price falls below average variable cost, the firm will shut down.)

In the long run, the firm adjusts its inputs so that its long-run marginal cost is equal to the market price. At this level of output, it is operating on a short-run marginal cost curve where short-run marginal cost is equal to price. As the long-run price changes, the firm gradually changes its mix of inputs to minimize cost. Thus, the long-run supply response is this adjustment from one set of short-run marginal cost

短期的我懂，可是长期的我不懂……

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关键词：复习题 平狄克 Maximizing adjustment Gradually 请教 狄克 复习题

长期时,生产规模可以调整,都是可变成本,没有固定成本.厂商的长期成本线由一系列短期的边际成本线组成,在每个价格上,厂商都选择其最合适的规模生产,因此长期成本曲线是一系列短期边际成本线的包络线.

但是，长期平均成本曲线才是短期平均成本曲线的包络线，长期边际成本曲线不是短期边际的包络线的。

我不懂的是，既然厂商也是选择其长期边际成本曲线和价格的交点的，为什么长期边际成本曲线不是长期供给曲线呢？

书上只是说了行业的长期供给曲线，并没有提到厂商的长期供给……

maybe i haven't make thorough clear of the problem.I will read some books about the problem and give you a reply.

where is this question in the book, can you show me?i have a chinese answer to the book written by a student but i do not have the original edition of english. can you mail to me? thanks a lot.

对于完全竞争厂商,经过长期竞争后,留下来的厂商必然都是成本最低的,而且该厂商只能在其最佳生产规模的最佳产量下生产,即LAC,LMC,SAC,SMC的交点生产.如果价格升高,则吸引新的厂商进入,而不会改变该厂商的生产规模和产量,如果价格降低,则一些厂商退出,使得行业供给减少,直到价格上升到该最低成本.因此完全竞争厂商的长期供给曲线并不是长期边际成本曲线.

对于垄断厂商的长期供给曲线,我现在还想不出来,哪位同学高见,请赐教.

可是周惠中的<微观经济学>的P171页,"完全竞争企业的长期供应曲线是边际成本曲线高出平均成本曲线的部分"(原话),两者有矛盾啊