announcements last lecture tonight 5-6 arts main lt thursday 9-10 muirhead main lt: fazeer exercise...
TRANSCRIPT
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Announcements
• Last lecture Tonight 5-6 ARTS Main LT
• Thursday 9-10 Muirhead Main LT: Fazeer Exercise Class on Production WorkSheet
• Marks on Test 2 hopefully by Friday
• CORRECT Version of Solution to Test 2 on Network (some minor 1 am errors in original)
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Tests in Last Week• 201 Test 3 on Production Worksheet
– Monday 11th Dec 5-6 Vaughan Jeffries
– see final previous years
• 201 TFU Final
– Thursday 14th Dec 5-6 Vaughan Jeffries
– see final previous years
• 203 Final (Two essays)
– Wednesday 13th 12-1 Vaughan Jeffries
• Will meet in Semester 2 to review test procedure
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So have 3 Distinct Problems
1) Maximise profits
Maxx1, x2 = P f (x1, x2) – w1x1 – w2x2
Gives factor demand functions
X1 = x1 (w1, w2)
X2 = x2 (w1, w2)
May not be well defined if there are constant returns to scale
Varian Appendix Ch 18
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2) Maximise subject to a constraintCxwx)s.t.wx,f(xQMax 221121xx 21
3) Minimise subject to a constraintQ)x,s.t.f(xxwxwMinC 212211
Problem 3) is the Dual of 2)
Called Duality Theory
Essentially allows us to look at problems in reverse and can often give very important insights.
Varian Appendix Ch19
Varian Appendix Ch 18
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Take Cobb-Douglas example of Problem 2:
e.g rK]wLcλ[LKQ ba
λwLbKdL
dQ 1ba (1)
(2)
rKwLcdλdQ (3)
λwL
bQ
λrLaKdK
dQ b1-a λrK
aQ
=0
=0
=0
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From 1) + 2)
λr
λw
KaQL
bQ
r
w
L
K
a
b
Lr
w.
b
aK
λwL
bQ λr
K
aQ
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Substitute into (3)
]r[wLC
wLb
awL C )wL
b
a(1
)wLb
ba(C
w
C
ba
bL
This is a the factor-demand function for L
Lr
w
b
a
Lr
w.
b
aK
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Strictly it is a Cost-constrained factor-demand function
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So maximising output subject to a cost or finance constraint
w
C
ba
bL
rK]wLcλ[LKQMAX baLK,
yields a solution for the use of the two factors:
r
C
ba
aK
Similar to demand theory
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So Production subject to a cost or a finance constraint is just like the usual consumption problem
• And a Cost-constrained factor-demand function is just a like normal Marshallian demand function in consumer theory
• …. with all its associated properties, adding-up, conditions, Cournot etc.
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But what about the third problem… That of minimising
cost subject to an output constraint?
• Again we will work through a Cobb-Douglas example to give you a flavour of the story.
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Cobb-Douglas Example of problem 3
dL
dC)1(
d
dC)3(
dK
dC)2(
1baLbKw
b1-a LaKr
L
Qbw
K
Qar
)LK(rKwLC MIN baLK, Q
baLKQ
=0
=0
=0
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From (1) + (2)
KQλa
LQλb
r
w
L
K
a
b
r
w
Lr
w.
b
aK
L
Qbw
K
Qλar
Which is precisely the same condition as before, So both problems tell us that we should use K and L according to the same rule
(4)
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But now comes the different bit!baLKQThe constraint (3) was
ba L Q Lr
w.
b
a
and Lr
w.
b
aK
baa
Lr
w
b
aQ
(4)
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But if have CRS a+b=
baa
Lr
w
b
aQ
Lr
w
b
aQ
a
1
Qw
r
a
bL
a
[note we invert the bracket when we bring it to the other side]
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This is a different factor demand function to the ‘normal’, ‘Marshallian-like’ demand function we had earlier.
What does it correspond to?
Qw
r
a
bL
a
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L
K
Q
We are adjusting Costs to get to best point on target isoquant given w/r
If have different w/r, have differently sloped line, tangent to
Q
w0/r0
So picking out points of tangency along isoquant
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This is a different factor demand function to the ‘normal’, ‘Marshallian-like’ demand function we had earlier.
This is essentially a Hicksian Factor demand curve
It is a Quantity-Constrained CONDITIONAL factor-demand curve
Qw
r
a
bL
a
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baLKQThe constraint (3) was
baK Q
and Kw
r.
a
bL
bba
w
r
a
bKQ
Similarly we can solve for K. The easiest way to do this is to go back to the FOC
Kw
r
a
b
Inverting (4) above
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b
w
r
a
bKQ
Q
r
w
b
aK
b
[note we again invert the bracket when we bring it to the other side]
And with CRS a + b=1 as before,
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So Minimising Cost subject to an output constraint
yields a solution for the use of the two factors:
Conditional on the level of output.
Similar to Hicksian demand in consumer theory
)LK(rKwLC MIN baLK, Q
Qw
r
a
bL
a
Q
r
w
b
aK
b
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Note, since
w
Q
aw
bra
dw
dLa
So conditional factor-demand functions always slope down
Q Constant – so no ‘income’ type effect
Qw
r
a
bL
a
< 0
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Cost Function
• But the objective of this exercise is not just to derive conditional factor demands, but rather to derive a cost function
• To do this we return to the original
• C= wL + rK
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Note can now formalise the cost function for the item C = wL +rK
rwC
bbb1
aaa1
b
aQwr
a
bQrw
b
br
awQ
a
aw
brQ
b
b
ba
a
a
b
aQ
r
wr
a
bQ
w
rw
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Qrw C ab
Qr w2 C 1/21/21/2b1/2,a e.g.
bba
aab
b
aQwr
a
bQrw
baab
b
a
a
bQrw
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Qr w2 C 1/21/2
L
K
1/21/2LK Q
Notice C-D production function takes the
form:
w
r
While its cost function for a given quantity Q
takes the form:
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ba
1ba
a
ba
b
Qrk w C
Now we assumed earlier that there were CRS and that a+b=1
No reason why it should be of course, and more generally the Cobb-Douglas version of the cost function is:
Where k is a constant that depends on a and b
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TC
AC
Q
Q
ba
1ba
a
ba
b
Qrkw C
ba
b-a-1ba
a
ba
b
Qrkw AC
Long Run total and average cost functions
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Qrkw C ab
TC
AC abrkw AC
If a + b =1, CRS and constant LR average costs
Q
Q
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TC
AC
If a + b <1, DRS and increasing LR average costs
Q
Q
ba
1ba
a
ba
b
Qrkw C
ba
b-a-1ba
a
ba
b
Qrkw AC
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TC
AC
If a + b >1, IRS and decreasing LR average costs
Q
Q
ba
1ba
a
ba
b
Qrkw C
ba
b-a-1ba
a
ba
b
Qrkw AC
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What about the Short-Run?
• Derivation of short-run costs from an isoquant map
– Recall in SR Capital stock is fixed
• Derivation of short-run costs from an isoquant map
– Recall in SR Capital stock is fixed
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fig
Un
its o
f ca
pita
l (K
)
O
Units of labour (L)
TC1
TC4
TC7
100
400
700
Deriving a SDeriving a SRACRAC curve from an isoquant map curve from an isoquant map
L0
K0
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TC
AC
SR TC and AC curve lies above LR costs
Q
Q
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The Short Run Cost Functionba
LKQ Note
So the actual L employed at any point is :
b
1
aK
QL
So short run costs are:
KrwLSRTC KrK
Q w
b
1
a
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The Short Run Cost Function for a=b=1/2
Since
KrK
Q w
b
1
a
C
KrK
Qw
2
1/2
KrQ
K
w 2
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The Short Run Marginal Cost Function for a=b=1/2
Since
We take the derivative to find MC
KrQK
w 2 C
QK
w2
dQ
dC
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Under Perfect Competition MC=MR=P
Which gives us the short run supply curve of the firm i:
Kw
P
2
1QS
PQK
w2
Q
P MC=Qs
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Equilibrium :
To find an equilibrium then we have to set D =S
i
iMCS
D
P
Q
The remaining aspects of any production problem ask you to apply first year ideas to a specific problem
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Gereralised version of Problem 3
Minimise cost : wL + w2 K
s.t. f(x1, x2) =Q
)]x,f(xQλ[xwxwCMin 212211xx21
0)x,f(xQdλdC
0dxdf
λwdxdC
0dxdf
λwdxdC
21
2
2
2
1
1
1
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3 EQNS – 3 unknowns x1, x2,
So solve for ‘Quantity Constrained’
Conditional factor demands
X1 = x1 (w1, w2, )
X2 = x2 (w1, w2, )