7.10 : 短期与长期：平均成本

In the short run, labor is variable while capital is fixed.

As a result, the Short-run average total cost or ATC_SR tends to be higher because fixed inputs limit optimization.

The Long Run Average Total Cost, or ATC_LR is the cost per unit when all inputs are variable. This input adjustment allows for economies of scale and lower cost per unit of output.

The minimum point of nearly all the possible short run average total cost curves cannot touch the long run average total cost curve. Indeed, only one short run average total cost curve can possibly share the same minimum point. This is because the output levels that minimize average total cost in the short run, with fixed capital, can be produced at a lower cost if the capital inputs are variable.

Consider a software company with a fixed capacity of servers and office space in the short run. Despite growing product demand, the company cannot immediately expand its infrastructure as it has limited resources.

But it can invest in more servers and office space in the long run. This allows the company to have lower Long run average total cost.