1. **Problem Statement:** We want to understand the conditions in a competitive market where firms earn economic profits and whether this situation can persist in the long run. 2. **Key Concepts and Formulas:** - In perfect competition, firms are price takers, so $MR = AR = P$ (Marginal Revenue = Average Revenue = Price). - Economic profit occurs when Price $P$ is greater than Average Total Cost $ATC$ at the profit-maximizing quantity $Q^*$. - The profit-maximizing output is where $MC = MR$. 3. **Graph Explanation:** - The graph has three curves: $MC$ (Marginal Cost), $ATC$ (Average Total Cost), and a horizontal line $MR = AR = P$. - The equilibrium point $E$ is where $MC = MR = P$. - At $Q^*$, the vertical distance between $P$ and $ATC$ represents the economic profit per unit. 4. **Why Economic Profit Occurs:** - Since $P > ATC$ at $Q^*$, the firm earns positive economic profit. - This attracts new firms to enter the market. 5. **Long-Run Outcome:** - Entry of new firms increases market supply, driving the price down. - Price falls until $P = ATC$ at the new equilibrium. - Economic profits are eliminated in the long run; firms earn zero economic profit. 6. **Conclusion:** - Economic profits can exist in the short run but cannot be maintained in the long run in a perfectly competitive market due to free entry and exit. --- **中英对照解答：** 1. **问题陈述：** 我们要理解在完全竞争市场中，企业获得经济利润的条件，以及这种情况是否能长期维持。 2. **关键概念和公式：** - 在完全竞争中，企业是价格接受者，因此 $MR = AR = P$（边际收益=平均收益=价格）。 - 当价格 $P$ 大于平均总成本 $ATC$ 时，企业获得经济利润。 - 利润最大化产量满足 $MC = MR$。 3. **图形说明：** - 图中有三条曲线：$MC$（边际成本）、$ATC$（平均总成本）和一条水平线 $MR = AR = P$。 - 平衡点 $E$ 是 $MC = MR = P$ 的交点。 - 在 $Q^*$ 处，$P$ 与 $ATC$ 之间的垂直距离表示单位经济利润。 4. **经济利润产生原因：** - 由于 $P > ATC$，企业获得正的经济利润。 - 这会吸引新企业进入市场。 5. **长期结果：** - 新企业进入增加市场供给，导致价格下降。 - 价格下降直到 $P = ATC$，经济利润消失。 6. **结论：** - 经济利润在短期内存在，但由于自由进入和退出，长期内无法维持，企业只能获得零经济利润。