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EC2010-Intermediate Economics
Microeconomics Module
Lecturer: Martín Paredes
Trinity College Dublin
Department of Economics
Hilary Term 2007
SOLUTIONS FOR ASSIGNMENT # 6
1. Chapter 7, Review Question # 8
Giffen goods arise when the income effect is so severely negative that it offsets the
substitution effect. This can happen because in consumer choice, income was an exogenous
variable – therefore, changes in price affect both the relative substitutability of goods (via the
tangency condition) as well as the consumer’s purchasing power (via the budget constraint).
By contrast, in the cost minimization problem output is exogenous while the expenditure is
the objective function. Thus, a change in an input price affects only the relative
substitutability of inputs (via the tangency condition) – there is no corresponding effect on
the production constraint, since prices do not appear there. So while there is a “substitution
effect” in cost minimization, there is no corresponding “income effect” as in consumer
choice. Therefore, increases in input prices will always lead to decreases in the use of that
input (except at corner solutions, where there might be no change). So there cannot be a
Giffen input.
2. Chapter 7, Review Question # 9
Assuming quantity is fixed, the short-run demand for a variable input would equal its longrun demand if the level of the fixed input in the short run was cost minimizing for the
quantity of output being produced in the long run.
3. Chapter 7, Problem # 7.7
The tangency condition implies
K
L
10 L  K
10 
1
Substituting into the production function yields
121, 000  LK
121, 000  L(10 L)
121, 000  10 L2
12,100  L2
110  L
Since K  10L , K  1,100 . The cost-minimizing quantities of labor and capital to
produce 121,000 airframes are K  1,100 and L  110 .
4. Chapter 7, Problem # 7.8
a)
1.2
1
K
0.8
0.6
0.4
0.2
0
0
2
4
6
8
10
12
L
K and L are perfect substitutes, meaning that the production function is linear and
the isoquants are straight lines. We can write the production function as Q =
10,000K + 1000L, where Q is the number of workers for whom payroll is
processed.
b)
If r  5 and w  7.50 , the slope of a typical isocost line will be 7.5/ 5.0  1.5 .
This is steeper than the isoquant implying that the firm will employ only
computer time ( K ) to minimize cost. The cost minimizing combination is K  1
and L  0 . This outcome can be seen in the graph below. The isocost lines are
the dashed lines.
2
2.5
2
Optimum
K
1.5
1
0.5
0
0
2
4
6
8
10
12
L
The total cost to process the payroll for 10,000 workers will be TC  5(1)  7.5(0)  5 .
c)
The firm will employ clerical time only if MPL / w > MPK / r. Thus we need 0.1 /
7.5 > 1/r or r > 75.
5. Chapter 7, Problem # 7.11
a)
First, note that this production function has diminishing MRSL,K. The tangency
condition would imply that 1/ 2 L  1/ 50 or L = 625. Substituting this back into
the production function we see that K = 10 – 25 = –15. Since the firm cannot use a
negative amount of capital, the tangency condition is not valid in this case.
Looking at the corner with K = 0, since Q = 10 the firm requires L = Q2 = 100
units of labor. At this point, MPL / w = (1/20)/1 = 0.05 > MPK / r = 1/50 = 0.02.
Since the marginal product per dollar is higher for labor, the firm will use only
labor and no capital.
MPL MPK
, or 2 L  r.

w
r
Thus L = 0.25r2. From the production constraint K = Q  L = 10 – 0.5r. So if
K > 0 then we must have 10 – 0.5r > 0, or r < 20.
b)
The firm will use a positive amount of capital when
c)
Again, using the tangency condition we must have 2 L  r. Therefore, since r =
50, L = 625. From the production constraint, the input demand for capital is K =
Q  L = Q – 25. So if K > 0 then we must have Q > 25.
3
6. Chapter 7, Problem # 7.12
No, these are not valid input demand curves. In both cases the quantity of the input is
positively related to the input’s price. Such upward-sloping input demand curves cannot
exist.
7. Chapter 7, Problem # 7.18
With just two inputs, there is no tangency condition to worry about in the short run. To
find the short-run cost-minimizing quantity of labor, we need only solve the production
function for L in terms of Q and K :
1
Q  10KL3
This gives us:
L
Q3
1000 K
3
This is the cost-minimizing quantity of labor in the short run.
8. Chapter 8, Review Question # 2
When the price of one input increases, the isocost line for a particular level of total cost
will rotate in toward the origin. Assuming the isocost line was tangent to the isoquant for
the firm’s selected level of output, when the isocost line rotates it will no longer touch the
original isoquant. In order for an isocost line to reach a tangency with the original
isoquant, the firm would need to move to an isocost line associated with a higher level of
cost, i.e. an isocost line further to the northeast.
9. Chapter 8, Review Question # 3
If the price of a single input goes up leaving all other input prices the same and the level
of output constant, total cost will rise but by a smaller percentage than the increase in the
input price. This occurs because the firm will substitute away from the now relatively
more expensive labor to the now relatively less expensive other inputs. So, if the price of
4
labor rises by 20% holding all other input prices constant, total cost will rise by less than
20%.
If the prices of all inputs go up by the same percentage, total cost will rise by exactly that
same percentage. So, if input prices rise by 20%, total cost will also rise by 20%.
10. Chapter 8, Review Question # 9
If the average variable cost curve is flat, average variable cost is neither increasing nor
decreasing. Marginal cost will therefore be equal to average variable cost and the
marginal cost curve will therefore also be flat. Since average fixed cost is always
declining, and since average total cost is the vertical sum of average variable and average
fixed costs, average total cost must also be declining at all levels of Q if average variable
cost is constant. Graphically, average total cost will be declining and asymptotic to the
average variable cost curve.
11. Chapter 8, Problem # 8.5
From the total cost curve, we can derive the average cost curve, AC(Q)  40  10Q  Q 2 .
The minimum point of the AC curve will be the point at which it intersects the marginal
cost curve, i.e. 40  10Q  Q 2  40  20Q  3Q 2 . This implies that AC is minimized when
Q = 5. By definition, there are economies of scale when the AC curve is decreasing (i.e.
Q < 5) and diseconomies when it is rising (Q > 5).
12. Chapter 8, Problem # 8.8
a)
Each tricycle requires the purchase of three wheels at price PW and one frame at
price PF. Thus, TC(Q, PW, PF) = Q(3PW + PF).
b)
Three wheels and one frame are perfect complements in production. Thus the
production function is Q(F, W) = min{F, (1/3)W}. Notice that (F, W) = (1, 3)
yields Q = 1, (F, W) = (2, 6) yields Q = 2, etc.
5
13. Chapter 8, Problem # 8.11
As we saw in Chapter 7, linear production functions usually have corner solutions. In this
case, the firm will use only labor if
MRTS L , K 
Similarly, it will use only capital if
w
w r
, or 
r
3 5
w r
 .
3 5
If the firm does use labor, then it will use L 
uses capital it will use K 
Q
with a total cost of wQ/3. Similarly if it
3
Q
with a total cost of rQ/5.
5
Therefore, the firm’s total cost curve can be expressed as TC  min{
w r
, }Q.
3 5
14. Chapter 8, Problem # 8.12
a)
From the production function we see that Q  5 L , so the amount of labor
required to produce Q is given by L 
Q2
. The short run total cost function is
25
 Q2 
C  25L  20 K  25    20(5)  100  Q2 .
 25 
b)
AC
20
10
Q
6