Chapter 7: The Costs of Production

CHAPTER 7
THE COST OF PRODUCTION

QUESTIONS FOR REVIEW

1. A firm pays its accountant an annual retainer of $10,000. Is this an economic cost?
Explicit costs are actual outlays. They include all costs that involve a monetary
transaction. An implicit cost is an economic cost that does not necessarily involve a
monetary transaction, but still involves the use of resources. When a firm pays an
annual retainer of $10,000, there is a monetary transaction. The accountant trades his
or her time in return for money. Therefore, an annual retainer is an explicit cost.
2. The owner of a small retail store does her own accounting work. How would you
measure the opportunity cost of her work?
Opportunity costs are measured by comparing the use of a resource with its alternative
uses. The opportunity cost of doing accounting work is the time not spent in other ways,
i.e., time such as running a small business or participating in leisure activity. The
economic, or opportunity, cost of doing accounting work is measured by computing the
monetary amount that the owner’s time would be worth in its next best use.
3. Please explain whether the following statements are true or false.
a. If the owner of a business pays himself no salary, then the accounting cost is zero,
but the economic cost is positive.
This is True. Since there is no monetary transaction, there is no accounting, or
explicit, cost. However, since the owner of the business could be employed elsewhere,
there is an economic cost. The economic cost is positive, and reflectsing the
opportunity cost of the owner’s time. The economic cost is the value of the next best
alternative, or the amount that the owner would earn if he took the next best job.
e.b. A firm that has positive accounting profit does not necessarily have positive 서식 있음: 글머리 기호 및 번호
economic profit. 매기기
True. Accounting profit considers only the explicit, monetary costs. Since there may
be some opportunity costs that were not considered fully realized as explicit
monetary costs, it is possible that when the opportunity costs are added in, economic
profit will become negative. This indicates that the firm’s resources are not being put
to their best use. Subtracting extra costs could make the profit negative, in economic
terms.
f.c. If a firm hires a currently unemployed worker, the opportunity cost of utilizing the 서식 있음: 글머리 기호 및 번호
worker’s services is zero. 매기기
False. The opportunity cost measures the value of the worker’s time, which is not
unlikely to be zero. Though the worker was temporarily unemployed, the worker still
possesses possesses certain skills, which have a value and make the opportunity cost
of hiring the worker greater than zero. In addition, since opportunity cost is the
equivalent of the worker’s next best option, it is possible that the worker might have
been able to get a better job that utilizes his skills more efficiently. Alternatively, the
worker could have been doing unpaid work, such as care of a child or elderly person
at home, which would have had a value to those receiving the service.
5.4. Suppose that labor is the only variable input to the production process. If the 서식 있음: 글머리 기호 및 번호
marginal cost of production is diminishing as more units of output are produced, what 매기기
can you say about the marginal product of labor (the variable input)?
The marginal product of labor must be rising increasing. The marginal cost of
production measures the extra cost of producing one more unit of output. If this cost
is diminishing, then it must be taking fewer units of labor to produce the extra unit
of output, since the extra cost refers to the extra cost of the labor. If fewer units of

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labor are required to produce a unit of output, then the marginal product (extra
output produced by an extra unit of labor) must be increasing. Note also, that
MC=w/MPL, so that if MC is diminishing then MPL must be increasing for any given
w.

5. Suppose a chair manufacturer finds that the marginal rate of technical substitution of
capital for labor in his production process is substantially greater than the ratio of the
rental rate on machinery to the wage rate for assembly-line labor. How should he alter his
use of capital and labor to minimize the cost of production?
To minimize cost, the manufacturer should use a combination of capital and labor so
the rate at which he can trade capital for labor in his production process is the same as
the rate at which he can trade capital for labor in external markets. The manufacturer
would be better off if he increased his use of capital and decreased his use of labor,
decreasing the marginal rate of technical substitution, MRTS. He should continue this
substitution until his MRTS equals the ratio of the rental rate to the wage rate. The
MRTS in this case is equal to MPK/MPL. As the manufacturer uses more K and less
L, the MPK will diminish and the MPL will increase, both of which will decrease the
MRTS until it is equal to the ratio of the input prices (rental rate on capital divided by
wage rate).
6. Why are isocost lines straight lines?
The isocost line represents all possible combinations of labor and capital that may be
purchased for a given total cost. The slope of the isocost line is the ratio of the input
prices of labor and capital. If input prices are fixed, then the ratio of these prices is
clearly fixed and the isocost line is straight. Only when the ratio or factor prices
change as the quantities of inputs change is the isocost line not straight.
7. Assume the marginal cost of production is increasing. Can you determine whether the
average variable cost is increasing or decreasing? Explain.
Marginal cost can be increasing while average variable cost is either increasing or
decreasing. If marginal cost is less (greater) than average variable cost, then each
additional unit is adding less (more) to total cost than previous units added to the total
cost, which implies that the AVC declines (increases). Therefore, we need to know
whether marginal cost is greater than average variable cost to determine whether the
AVC is increasing or decreasing.
8. Assume the marginal cost of production is greater than the average variable cost. Can
you determine whether the average variable cost is increasing or decreasing? Explain.
If the average variable cost is increasing (decreasing), then the last unit produced is
adding more (less) to total variable cost than the previous units did, on average.
Therefore, marginal cost is above (below) average variable cost. In fact, the point
where marginal cost exceeds average variable cost is also the point where average
variable cost starts to rise.
9. If the firm’s average cost curves are U-shaped, why does its average variable cost curve
achieve its minimum at a lower level of output than the average total cost curve?
Total cost is equal to fixed plus variable cost. Average total cost is equal to average
fixed plus average variable cost. When graphed, the difference between the U-shaped
total cost and average variable cost curves is the average fixed cost curve. Thus, falling
average variable cost and average fixed cost sum up to a falling average total cost curve.
Since average fixed cost continues to fall as more output is produced, average total cost
will continue to fall even after average variable cost has reached its minimum because
the drop in average fixed cost exceeds the increase in the average variable cost.
Eventually, the fall in average fixed cost becomes small enough so that the rise in
average variable cost causes average total cost to begin to rise.

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11.10. If a firm enjoys economies of scale up to a certain output level, and then cost 서식 있음: 글머리 기호 및 번호
increases proportionately with output, what can you say about the shape of the long-run 매기기
average cost curve?
When the firm experiences increasing returns to scale, its long-run average cost curve
is downward sloping. When the firm experiences constant returns to scale, its long-run
average cost curve is horizontal. If the firm experiences increasing returns to scale,
then constant returns to scale, its long-run average cost curve falls, then becomes
horizontal.
11. How does a change in the price of one input change the firm’s long-run expansion
path?
The expansion path describes the combination of inputs that the firm chooses to
minimize cost for every output level. This combination depends on the ratio of input
prices: if the price of one input changes, the price ratio also changes. For example, if
the price of an input increases, less of the input can be purchased for the same total
cost, and the intercept of the isocost line on that input’s axis moves closer to the origin.
Also, the slope of the isocost line, the price ratio, changes. As the price ratio changes,
the firm substitutes away from the now more expensive input toward the cheaper
input. Thus, the expansion path bends toward the axis of the now cheaper input.
12. Distinguish between economies of scale and economies of scope. Why can one be
present without the other?
Economies of scale refer to the production of one good and occur when proportionate
increases in all inputs lead to a more-than-proportionate increase in output.
Economies of scope refer to the production of more than one good and occur when joint
output is less costly than the sum of the costs of producing each good or service
separately. There is no direct relationship between increasing returns to scale and
economies of scope, so production can exhibit one without the other. See Exercise (14)
for a case with constant product-specific returns to scale and multiproduct economies of
scope.
16.13. Is the firm’s expansion path always a straight line? 서식 있음: 글머리 기호 및 번호
No. If the long run expansion path is a straight line this means that the firm always 매기기
uses capital and labor in the same proportion. If the capital labor ratio changes as
output is increased then the expansion path is not a straight line.
Also, in the short run the expansion path may be horizontal if capital is fixed.
17.14. What is the difference between economies of scale and returns to scale? 서식 있음: 글머리 기호 및 번호
Economies of scale measures the relationship between what happens to cost and 매기기
when outpuoutput, i.e., when output is doubled, does cost double, less then double, or
more than double. t is doubled. Returns to scale measures what happens to output
when all inputs are doubled.

EXERCISES
1.1. Joe quits his computer-programming job, where he was earning a salary of $50,000 서식 있음: 글머리 기호 및 번호
per year to start . He opens his own computer software business store in a building that 매기기
he owns and was previously renting out for $24,000 per year. In his first year of business
he has the following expenses: mortgage $18,000, salary paid to himself $40,000, rent, $0,
and other expenses $25,000. Find the accounting cost and the economic cost associated
with Joe’s computer software business.
The accounting cost represents the actual expenses, which are 18,000+$40,000+$0 +
$25,000=$8365,000. The economic cost includes accounting cost, but also takes into
account opportunity cost. Therefore, economic will include, in addition to accounting
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cost, an extra $246,000 because he Joe gave up $624,000 by not renting the building
($24,000-$18,000), and an extra $10,000 because he paid himself a salary gave up
$10,000 below market on his salary ($50,000-$40,000). Economic cost is then $99,000.

2.2. a. Fill in the blanks in the following table. 서식 있음: 글머리 기호 및 번호

매기기
Units of Fixed Variable Total Marginal Average Average Average
Output Cost Cost Cost Cost Fixed Variable Total Cost
Cost Cost
0 100 0 100 -- -- 0 --
1 100 25 125 25 100 25 125
2 100 45 145 20 50 22.5 72.5
3 100 57 157 12 33.3 19 52.3
4 100 77 177 20 25 19.25 44.25
5 100 102 202 25 20 20.4 40.4
6 100 136 236 34 16.67 22.67 39.3
7 100 170 270 34 14.3 24.3 38.6
8 100 226 326 56 12.5 28.25 40.75
9 100 298 398 72 11.1 33.1 44.2
10 100 390 490 92 10 39 49

b. Draw a graph that shows marginal cost, average variable cost, and average total
cost, with cost on the vertical axis and quantity on the horizontal axis.
Average total cost is u-shaped and reaches a minimum at an output of 7, based on
the above table. Average variable cost is u-shaped also and reaches a minimum at an
output of 3. Notice from the table that average variable cost is always below average
total cost. The difference between the two costs is the average fixed cost. Marginal
cost is first diminishing, to a quantity of 3 based on the table, and then increases as q
increases. Marginal cost should intersect average variable cost and average total
cost at their respective minimum points, though this is not accurately reflected in the
numbers in the table. If the specific functions had been given in the problem instead
of just a series of numbers, then it would be possible to find the exact point of
intersection between marginal and average total cost and marginal and average
variable cost. The curves are likely to intersect at a quantity that is not a whole
number, and hence are not listed in the above table.
3.3. A firm has a fixed production costs of $5,000 and a constant marginal cost of 서식 있음: 글머리 기호 및 번호
production of equal to $500 per unit produced. 매기기
a. What is the firm’s total cost function? Average cost?
The variable cost of producing an additional unit, marginal cost, is constant at $500, so
VC $500q
VC  $500q , and AVC    $500. Fixed cost is $5,000 and average
q q
$5,000
fixed cost is . The total cost function is fixed cost plus variable cost or
q
TC=$5,000+$500q. Average total cost is the sum of average variable cost and average
$5,000
fixed cost: ATC  $500  .
q

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b. If the firm wanted to minimize the average total cost, would it choose to be very
large or very small? Explain.
The firm should choose a very large output because average total cost will continue to
decrease as q is increased. As q becomes infinitely large, ATC will equal $500.

4. Suppose a firm must pay an annual tax, which is a fixed sum, independent of whether
it produces any output.
a. How does this tax affect the firm’s fixed, marginal, and average costs?
Total cost, TC, is equal to fixed cost, FC, plus variable cost, VC. Fixed costs do not
vary with the quantity of output. Because the franchise fee, FF, is a fixed sum, the
FC  VC
firm’s fixed costs increase by this fee. Thus, average cost, equal to , and
q
FC FF
average fixed cost, equal to , increase by the average franchise fee . Note
q q
that the franchise fee does not affect average variable cost. Also, because marginal
cost is the change in total cost with the production of an additional unit and because
the fee is constant, marginal cost is unchanged.
b. Now suppose the firm is charged a tax that is proportional to the number of items it
produces. Again, how does this tax affect the firm’s fixed, marginal, and average
costs?
Let t equal the per unit tax. When a tax is imposed on each unit produced, variable
costs increase by tq. Average variable costs increase by t, and because fixed costs are
constant, average (total) costs also increase by t. Further, because total cost increases
by t with each additional unit, marginal costs increase by t.
5. A recent issue of Business Week reported the following:
During the recent auto sales slump, GM, Ford, and Chrysler decided
it was cheaper to sell cars to rental companies at a loss than to lay off
workers. That’s because closing and reopening plants is expensive,
partly because the auto makers’ current union contracts obligate
them to pay many workers even if they’re not working.

When the article discusses selling cars “at a loss,” is it referring to accounting profit or
economic profit? How will the two differ in this case? Explain briefly.
When the article refers to the car companies selling at a loss, it is referring to
accounting profit. The article is stating that the price obtained for the sale of the
cars to the rental companies was less than their accounting cost. Economic profit
would be measured by the difference of the price with the opportunity cost of the cars.
This opportunity cost represents the market value of all the inputs used by the
companies to produce the cars. The article mentions that the car companies must
pay workers even if they are not working (and thus producing cars). This implies
 that the wages paid to these workers are sunk and are thus not part of the
opportunity cost of production. On the other hand, the wages would still be included
 in the accounting costs. These accounting costs would then be higher than the
opportunity costs and would make the accounting profit lower than the economic
profit.

6. Suppose the economy takes a downturn, and that labor costs fall by 50 percent and are
expected to stay at that level for a long time. Show graphically how this change in the
relative price of labor and capital affects the firm’s expansion path.

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Figure 7.6 shows a family of isoquants and two isocost curves. Units of capital are on
the vertical axis and units of labor are on the horizontal axis. (Note: In drawing this
figure we have assumed that the production function underlying the isoquants exhibits
constant returns to scale, resulting in linear expansion paths. However, the results do
not depend on this assumption.)
If the price of labor decreases while the price of capital is constant, the isocost curve
pivots outward around its intersection with the capital axis. Because the expansion
path is the set of points where the MRTS is equal to the ratio of prices, as the isocost
curves pivot outward, the expansion path pivots toward the labor axis. As the price of
labor falls relative to capital, the firm uses more labor as output increases.
Capital
Expansion path
before wage fall
4 Expansion path
after wage fall

Labor
1 2 3 4 5
Figure 7.6

8.7. The cost of flying a passenger plane from point A to point B is $50,000. The airline 서식 있음: 글머리 기호 및 번호
flies this route four times per day at 7am, 10am, 1pm, and 4pm. The first and last flights 매기기
are fulfilled l to capacity with 240 people. The second and third flights are only half full.
Find the average cost per passenger for each flight. Suppose the airline hires you as a
marketing consultant and wants to know which type of customer it should try to attract,
the off-peak customer (the middle two flights) or the rush-hour customer (the first and
last flights). What advice would you offer?
The average cost per passenger is $50,000/240 for the full flights and $50,000/120 for
the half full flights. The airline should focus on attracting more off-peak customers
in order to reduce the average cost per passenger on those flights. The average cost
per passenger is already minimized for the two peak time flights.
8. You manage a plant that mass produces engines by teams of workers using assembly
machines. The technology is summarized by the production function.
q = 5 KL

where q is the number of engines per week, K is the number of assembly machines, and L
is the number of labor teams. Each assembly machine rents for r = $10,000 per week and
each team costs w = $5,000 per week. Engine costs are given by the cost of labor teams
and machines, plus $2,000 per engine for raw materials. Your plant has a fixed
installation of 5 assembly machines as part of its design.
a. What is the cost function for your plant — namely, how much would it cost to
produce q engines? What are average and marginal costs for producing q engines?
How do average costs vary with output?

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K is fixed at 5. The short-run production function then becomes q = 25L. This

q
implies that for any level of output q, the number of labor teams hired will be L .
25
The total cost function is thus given by the sum of the costs of capital, labor, and raw
materials:
q
TC(q) = rK +wL +2000q = (10,000)(5) + (5,000)( ) + 2,000 q
25
TC(q)= 50,000 +2200q.
The average cost function is then given by:
TC(q) 50,000  2200q
AC(q)   .
q q
and the marginal cost function is given by:
TC
MC(q)   2200.
q
Marginal costs are constant and average costs will decrease as quantity increases
(due to the fixed cost of capital).
b. How many teams are required to produce 250 engines? What is the average cost
per engine?
q
To produce q = 250 engines we need labor teams L or L=10. Average costs are
25
given by
50,000  2200(250)
AC(q  250)   2400.
250
c. You are asked to make recommendations for the design of a new production
facility. What capital/labor (K/L) ratio should the new plant accommodate if it
wants to minimize the total cost of producing any level of output q?
We no longer assume that K is fixed at 5. We need to find the combination of K and
 L that minimizes costs at any level of output q. The cost-minimization rule is given
by

MPK MPL
= .
r w
To find the marginal product of capital, observe that increasing K by 1 unit increases
q by 5L, so MPK = 5L. Similarly, observe that increasing L by 1 unit increases Q by
5K, so MPL = 5K. Mathematically,

Q Q
MPK   5L and MPL   5K .
K L
Using these formulas in the cost-minimization rule, we obtain:
5L 5K K w 5000 1
     .
r w L r 10,000 2
The new plant should accommodate a capital to labor ratio of 1 to 2. Note that the
current firm is presently operating at this capital-labor ratio.
9. The short-run cost function of a company is given by the equation TC=200+55q, where
TC is the total cost and q is the total quantity of output, both measured in thousands.

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a. What is the company’s fixed cost?

When q = 0, TC = 200, so fixed cost is equal to 200 (or $200,000).

b. If the company produced 100,000 units of goods, what is its average variable cost?
With 100,000 units, q = 100. Variable cost is 55q = (55)(100) = 5500 (or $5,500,000).
TVC $5500
Average variable cost is   $55, or $55,000.
q 100
c. What is its marginal cost per unit produced?
With constant average variable cost, marginal cost is equal to average variable cost,
$55 (or $55,000).
d. What is its average fixed cost?
TFC $200
At q = 100, average fixed cost is   $2 or ($2,000).
q 100
e. Suppose the company borrows money and expands its factory. Its fixed cost rises by
$50,000, but its variable cost falls to $45,000 per 1,000 units. The cost of interest (i)
also enters into the equation. Each one-point increase in the interest rate raises
costs by $3,000. Write the new cost equation.
Fixed cost changes from 200 to 250, measured in thousands. Variable cost decreases
from 55 to 45, also measured in thousands. Fixed cost also includes interest charges: 3i.
The cost equation is
C = 250 + 45q + 3i.
10. A chair manufacturer hires its assembly-line labor for $30 an hour and calculates that
the rental cost of its machinery is $15 per hour. Suppose that a chair can be produced
using 4 hours of labor or machinery in any combination. If the firm is currently using 3
hours of labor for each hour of machine time, is it minimizing its costs of production? If
so, why? If not, how can it improve the situation? Graphically illustrate the isoquant and
the two isocost lines, for the current combination of labor and capital and the optimal
combination of labor and capital.
If the firm can produce one chair with either four hours of labor or four hours of capital,
machinery, or any combination, then the isoquant is a straight line with a slope of -1
and intercept at K = 4 and L = 4, as depicted in figure 7.10.
30
The isocost line, TC = 30L + 15K has a slope of 
 2 when plotted with capital on
15
TC TC
the vertical axis and has intercepts at K  and L  . The cost minimizing
15 30
point is a corner solution, where L = 0 and K = 4. At that point, total cost is $60. Two
isocost lines are illustrated on the graph. The first one is further from the origin and
represents the higher cost ($105) of using 3 labor and 1 capital. The firm will find it
optimal to move to the second isocost line which is closer to the origin, and which
represents a lower cost ($60). In general, the firm wants to be on the lowest isocost line
possible, which is the lowest isocost line that still intersects the given isoquant.

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Capital

isocost lines
4

isoquant

Labor

15.Figure 7.10 서식 있음: 글머리 기호 및 번호

1 1 매기기
11. Suppose that a firm’s production function is q  10L K . The cost of a unit of labor is
2 2

$20 and the cost of a unit of capital is $80.

a. The firm is currently producing 100 units of output, and has determined that the
cost-minimizing optimal quantities of labor and capital are 20 and 5 respectively.
Graphically illustrate this situation on a graph using isoquants and isocost lines.
The isoquant is convex. The optimal quantities of labor and capital are given by the
point where the isocost line is tangent to the isoquant. The isocost line has a slope of
1/4, given labor is on the horizontal axis. The total cost is TC=$20*20+$80*5=$800,
so the isocost line has the equation $800=20L+80K. On the graph, the optimal point
is point A.
capit al

point A

isoquant

labor

b. The firm now wants to increase output to 140 units. If capital is fixed in the short
run, how much labor will the firm requireneed? Illustrate this point on your
graph and find the new cost.
1 1

The new level of labor is 39.2. To find this, use the production function q  10L2 K 2
and substitute 140 in for output and 5 in for capital. The new cost is
TC=$20*39.2+$80*5=$1184. The new isoquant for an output of 140 is above and to
the right of the old isoquant for an output of 100. Since capital is fixed in the short
run, the firm will move out horizontally to the new isoquant and new level of labor.
This is point B on the graph below. This is not likely to be the cost minimizing point.
Given the firm wants to produce more output, they are likely to want to hire more
capital in the long run. Notice also that there are points on the new isoquant that
are below the new isocost line. These points all involve hiring more capital.

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capit al
point C
point B

labor

c. Graphically identify the optimal cost-minimizing level of capital and labor in the
long run if the firm wants to produce 140 units.
This is point C on the graph above. When the firm is at point B they are not
minimizing cost. The firm will find it optimal to hire more capital and less labor and
move to the new lower isocost line. All three isocost lines above are parallel and have
the same slope.
K
d. If the marginal rate of technical substitution is , find the optimal level of capital
L
and labor required to produce the 140 units of output.

Set the marginal rate of technical substitution equal to the ratio of the input costs so
K 20 L
that   K  . Now substitute this into the production function for K, set q
L 80 4
1
1
L2
equal to 140, and solve for L: 140 10L
4   L  28,K  7.
2
The new cost is

TC=$20*28+$80*7 or $1120.
12. A computer company’s cost function, which relates its average cost of production AC
to its cumulative output in thousands of computers Q and its plant size in terms of
thousands of computers produced per year q, within the production range of 10,000 to
50,000 computers is given by
AC = 10 - 0.1Q + 0.3q.
a. Is there a learning curve effect?
The learning curve describes the relationship between the cumulative output and the
inputs required to produce a unit of output. Average cost measures the input
requirements per unit of output. Learning curve effects exist if average cost falls with
increases in cumulative output. Here, average cost decreases as cumulative output, Q,
increases. Therefore, there are learning curve effects.
b. Are there economies or diseconomies of scale?
Economies of scale can be measured by calculating the cost-output elasticity, which
measures the percentage change in the cost of production resulting from a one
percentage increase in output. There are economies of scale if the firm can double its
output for less than double the cost. There are economies of scale because the
average cost of production declines as more output is produced, due to the learning
effect.

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c. During its existence, the firm has produced a total of 40,000 computers and is
producing 10,000 computers this year. Next year it plans to increase its production
to 12,000 computers. Will its average cost of production increase or decrease?
Explain.
First, calculate average cost this year:
AC1 = 10 - 0.1Q + 0.3q = 10 - (0.1)(40) + (0.3)(10) = 9.

Second, calculate the average cost next year:

AC2 = 10 - (0.1)(50) + (0.3)(12) = 8.6.

(Note: Cumulative output has increased from 40,000 to 50,000.) The average cost will
decrease because of the learning effect.
13. Suppose the long-run total cost function for an industry is given by the cubic
2 3
equation TC = a + bQ + cQ + dQ . Show (using calculus) that this total cost function is
consistent with a U-shaped average cost curve for at least some values of a, b, c, d.
To show that the cubic cost equation implies a U-shaped average cost curve, we use
algebra, calculus, and economic reasoning to place sign restrictions on the parameters
of the equation. These techniques are illustrated by the example below.
First, if output is equal to zero, then TC = a, where a represents fixed costs. In the
short run, fixed costs are positive, a > 0, but in the long run, where all inputs are
variable a = 0. Therefore, we restrict a to be zero.
Next, we know that average cost must be positive. Dividing TC by Q:
2
AC = b + cQ + dQ .
This equation is simply a quadratic function. When graphed, it has two basic shapes: a
U shape and a hill shape. We want the U shape, i.e., a curve with a minimum
(minimum average cost), rather than a hill shape with a maximum.
At the minimum, the slope should be zero, thus the first derivative of the average cost
curve with respect to Q must be equal to zero. For a U-shaped AC curve, the second
derivative of the average cost curve must be positive.
The first derivative is c + 2dQ; the second derivative is 2d. If the second derivative is
to be positive, then d > 0. If the first derivative is equal to zero, then solving for c as a
function of Q and d yields: c = -2dQ. If d and Q are both positive, then c must be
negative: c < 0.
To restrict b, we know that at its minimum, average cost must be positive. The
minimum occurs when c + 2dQ = 0. We solve for Q as a function of c and d:
c
Q  0 . Next, substituting this value for Q into our expression for average cost,
2d
and simplifying the equation:
2
c  c  , or
AC  b  cQ  dQ  b  c
2
 d
2d  2d 

c2 c2 2c 2 c2 c2
AC  b   b   b  0.
2d 4d 4d 4d 4d
c2
implying b 2
. Because c >0 and d > 0, b must be positive.
4d
In summary, for U-shaped long-run average cost curves, a must be zero, b and d must be
2
positive, c must be negative, and 4db > c . However, the conditions do not insure that
marginal cost is positive. To insure that marginal cost has a U shape and that its
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 Chapter 7: The Costs of Production

minimum is positive, using the same procedure, i.e., solving for Q at minimum marginal
2
cost c / 3d , and substituting into the expression for marginal cost b + 2cQ + 3dQ , we find
2
that c must be less than 3bd. Notice that parameter values that satisfy this condition
2
also satisfy 4db > c , but not the reverse.
2 3
For example, let a = 0, b = 1, c = -1, d = 1. Total cost is Q - Q + Q ; average cost is
2 2
1 - Q + Q ; and marginal cost is 1 - 2Q + 3Q . Minimum average cost is Q = 1/2 and
minimum marginal cost is 1/3 (think of Q as dozens of units, so no fractional units are
produced). See Figure 7.13.

Costs

2
MC

1 AC

0.33 0.67 0.83 1.00 Quantity

0.17 0.50
in Dozens
Figure 7.13
*14. A computer company produces hardware and software using the same plant and
labor. The total cost of producing computer processing units H and software programs S
is given by
TC = aH + bS - cHS,
where a, b, and c are positive. Is this total cost function consistent with the presence of
economies or diseconomies of scale? With economies or diseconomies of scope?
There are two types of scale economies to consider: multiproduct economies of scale and
product-specific returns to scale. From Section 7.5 we know that multiproduct
economies of scale for the two-product case, SH,S, are

TC H, S
SH ,S 
H MCH   S MCS 
where MCH is the marginal cost of producing hardware and MCS is the marginal cost of
producing software. The product-specific returns to scale are:
TC H, S  TC 0, S
SH  and
HMCH 
TC H, S  TC H,0
SS 
SMCS 

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 Chapter 7: The Costs of Production

where TC(0,S) implies no hardware production and TC(H,0) implies no software

production. We know that the marginal cost of an input is the slope of the total cost
with respect to that input. Since
TC  a cSH  bS  aH b  cHS,
we have MCH = a - cS and MCS = b - cH.

Substituting these expressions into our formulas for SH,S, SH, and SS:
aH  bS  cHS
SH ,S  or
Ha  cS  Sb  cH
aH  bS  cHS
SH ,S   1 , because cHS > 0. Also,
Ha  Sb  2cHS
aH bS  cHS   bS
SH  , or
Ha  cS
aH cHS  a  cS
SH    1 and similarly
Ha  cS a  cS
aH bS  cHS  aH
SS   1.
Sb  cH
There are multiproduct economies of scale, SH,S > 1, but constant product-specific
returns to scale, SH = SC = 1.
Economies of scope exist if SC > 0, where (from equation (7.8) in the text):

TC H,0  TC 0, S  TC H,S 

Sc  , or,
TC H, S
aH  bS  aH  bS cHS 
Sc  , or
TC H, S
cHS
Sc   0.
TC H,S
Because cHS and TC are both positive, there are economies of scope.

96