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Mutual Relationship Between Short-Run and Long-Run Cost Curves
1. Envelope Relationship
o LRAC is the envelope of all SRACs.
o At low output, a small plant (SRAC1) is cheapest.
o At medium output, a medium plant (SRAC2) is cheapest.
o At high output, a large plant (SRAC3) is cheapest.
2. Flexibility
o In the short run, the firm is stuck with a given plant size, so costs may be
higher.
o In the long run, the firm can adjust plant size, so costs are minimized.
3. Shape Difference
o SRAC curves are more sharply U-shaped due to the law of variable
proportions.
o LRAC is flatter because it reflects economies and diseconomies of scale.
4. Marginal Cost Relationship
o In both short and long run, MC curves intersect AC curves at their minimum
points.
o This shows the universal principle: when marginal is below average, it pulls
average down; when above, it pushes average up.
Story-Like Example
Think again of your bakery:
In the short run, you have one oven. If demand rises, you can hire more helpers and
buy more flour, but after a point, the oven becomes overcrowded, and costs rise
steeply. Thats your SRAC.
In the long run, you can buy a second oven or move to a bigger shop. Now you can
produce more at lower average cost. Thats your LRAC.
If demand keeps rising, you may open a factory with ten ovens. But managing so
many workers may create inefficiencies, raising costs again. Thats the upward slope
of LRAC.
Conclusion
The traditional theory of costs gives us a clear picture of how firms behave in the short run
and long run.
In the short run, costs are shaped by fixed commitments and the law of variable
proportions, giving us TFC, TVC, TC, AFC, AVC, AC, and MC curves.
In the long run, all inputs are variable, and costs are shaped by economies and
diseconomies of scale, giving us LRAC and LRMC.
The mutual relationship is that LRAC is the envelope of SRACs, showing the least
cost for each level of output.