# 西经高鸿业第三版lecture5

LECTURE5 成本论

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MC (Q) ? ?TC (Q) ?Q

Cost Curves for a Firm
Cost 400
(\$ per year)

Total cost is the vertical sum of FC and VC.

TC VC
Variable cost increases with production and the rate varies with increasing & decreasing returns.

300

200

100 50
0 1 2 3 4 5 6 7 8 9

Fixed cost does not vary with output

FC
10 11 12 13 Output

Cost Curves for a Firm
Cost
(\$ per unit)

100

MC
75

50

ATC AVC

25

AFC
0 1 2 3 4 5 6 7 8 9 10 11 Output (units/yr.)

Cost Curves for a Firm
The line drawn from the origin to the tangent of the variable cost curve:
? ? ?

P
400

TC VC

Its slope equals AVC The slope of a point on VC equals MC Therefore, MC = AVC at 7 units of output (point A)

300

200

A
100

FC
1 2 3 4 5 6 7 8 9 10 11 12

0

13 Output

Cost Curves for a Firm
Unit Costs
? ?
Cost
(\$ per unit)

?

AFC falls continuously When MC < AVC or MC < ATC, AVC & ATC decrease When MC > AVC or MC > ATC, AVC & ATC increase

100

MC
75

50

ATC AVC

25

AFC
0 1 2 3 4 5 6 7 8 9 10

11

Output (units/yr.)

Cost Curves for a Firm
Unit Costs
?
Cost
(\$ per unit)

?

MC = AVC and ATC at minimum AVC and ATC Minimum AVC occurs at a lower output than minimum ATC due to FC

100

MC
75

50

ATC AVC

25

AFC
0 1 2 3 4 5 6 7 8 9 10

11

Output (units/yr.)

?

?

TC (Q) ? TVC (Q) ? TFC ? w ? L(Q) ? TFC

dTC dL MC ? ?w ?0 dQ dQ 1 MC ? w ? MPL

TVC L 1 AVC ? ?w ?w Q Q APL

? ?

? ?

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? LMC > LAC，则LAC递增； ? LMC < LAC，则LAC递减；
? LMC = LAC，此时LAC最小。

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Long-Run Average and Marginal Cost
Cost (\$ per unit of output

LMC LAC

A

Output

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Ec ? (?C / C ) /(?Q / Q) Ec ? (?C / ?Q) /(C / Q) ? MC/AC

?

? EC < 1: MC < AC，规模经济递减 ? EC = 1: MC = AC，规模经济恒定； ? EC > 1: MC > AC，规模不经济递增。

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LAC的移动：
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Long-Run Cost with Economies and Diseconomies of Scale
Cost (\$ per unit of output \$10 SAC1 SAC2

SAC3

LAC

A B

\$8

SMC1

SMC3 SMC2

LMC

If the output is Q1 a manager would chose the small plant SAC1 and SAC \$8. Point B is on the LAC because it is a least cost plant for a given output.

Q1

Output

A Firm’s Expansion Path
Capital per year 150 \$3000 Isocost Line
The expansion path illustrates the least-cost combinations of labor and capital that can be used to produce each level of output in the long-run.

\$2000 Isocost Line

Expansion Path
C

100 75

B
50 A 25
200 Unit Isoquant 300 Unit Isoquant

50

100

150

200

300

Labor per year

The Inflexibility of Short-Run Production
Capital per year

E C
The long-run expansion path is drawn as before..

Long-Run Expansion Path

A K2 P K1 Q1
L1 L2 B L3 D F Labor per year

Short-Run Expansion Path

Q2

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