Question 1

What is the difference between short-run and long-run marginal cost?

Accepted Answer

Short-run marginal cost (MCSR) reflects the additional cost of producing one more unit when at least one input is fixed. Long-run marginal cost (MCLR) is the cost of producing one additional unit when all inputs are variable, allowing firms to adjust operations and achieve greater efficiency and lower costs per additional unit.

Question 2

Why does short-run marginal cost initially decline before rising?

Accepted Answer

Short-run marginal cost initially declines due to increasing marginal returns as production increases. However, beyond a certain point, diminishing returns set in, requiring additional resources to produce each extra output unit, causing marginal cost to rise and creating the characteristic U-shaped curve.

Question 3

How does the long-run marginal cost curve differ in shape from the short-run curve?

Accepted Answer

The long-run marginal cost curve tends to be flatter and smoother than the short-run curve. Because firms can adjust all inputs in the long run, they overcome diminishing returns more effectively and achieve economies of scale, allowing long-run marginal cost to remain relatively stable or decrease over a wider range of output levels.

Question 4

Where does the short-run marginal cost curve intersect the short-run average total cost curve?

Accepted Answer

Each short-run marginal cost curve crosses its corresponding short-run average total cost curve at the minimum point of the average total cost curve. This intersection represents the most efficient production level in the short run, where average costs are lowest.

Question 5

How is the long-run marginal cost curve constructed from short-run curves?

Accepted Answer

The long-run marginal cost curve is determined by identifying points where short-run average total cost curves intersect the long-run average total cost curve, then marking corresponding quantity points on the respective short-run marginal cost curves. Connecting these points across different output levels yields the long-run marginal cost curve.

Question 6

What is the relationship between long-run marginal cost and long-run average total cost curves?

Accepted Answer

When long-run average total cost is falling, the long-run marginal cost curve lies below it. When long-run average total cost is rising, the long-run marginal cost curve lies above it. Both curves intersect at the minimum point of the long-run average total cost.

Question 7

Why can firms achieve lower marginal costs in the long run compared to the short run?

Accepted Answer

In the long run, firms have flexibility to adjust all inputs, overcoming the constraints of fixed factors that exist in the short run. This input flexibility enables firms to achieve economies of scale more effectively, resulting in lower and more stable marginal costs per output unit across a wider production range.

短期与长期：边际成本

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