有人懂這條econ問題嗎?

For long run the cost curve look like(_____), this apply for any market structure. In short run even in perfect competition market firm can made super normal profit. But in long run they earn normal profit.

For other market structure the answer is not straight forward. The answer can be yes or no depend on what you call profitable, if it means normal profit it can be both yes or not, but super normal profit is not.

The reason is that the market is in equilibrium, but comer will upset the market and there will be disequilibrium. To fright for the market share and profit there must be a loser so the the market will return equilibrium, suppose not change in demand of course. So if the new comer fail they made lost, if the new comer success they made normal profit in the long run and may be supernormal profit in the short run.

In long run (in perfect competition), everybody has no difficulty in obtaining information. Therefore, no transaction cost exists. Every firms in the industry will produce until MR = MC = P. The last one product will have no gain or loss.
So under the long run, the new entrant will gain nothing because of no transaction cost.

2006-11-25 15:49:03 補充：
Please check my other reply in related to economic, you can learn more about it.