Econ 604 - Virginia Commonwealth University



Econ 604. Suggested Repsonses

Spring, 2006

Problem Set #3. Chapter 3, Problems 3.2, 3.4, 3.5, 3.7

3.2. Suppose the utility function for two goods, X and Y, has the Cobb-Douglas form

utility = U(X,Y) = (XY)1/2

a. Graph the U=10 indifference curve associated with this utility function

When U=10, we have

100 = XY

Or

Y = 100/X

Thus

dY/dX = - 100/X2

[pic]

b. If X = 5, what must Y equal to be on the U=10 indifference curve? What is the MRS at this point?

|X |Y |U |MRS |

|5 |20 |10 |4 |

|10 |10 |10 |1 |

|15 |6.666667 |10 |0.44 |

|20 |5 |10 |¼ |

With X=5, MRS = 100/25 = 4. For reference, I also list other values.

Reading down in the above table observe that as we move from the consumption of relatively few X’s to relatively more, the MRS falls. That is, you must give up progressively fewer units of Y to gain constant increments of X – a diminishing MRS that indicates a preference for a mix of X and Y.

c. In general, develop an expression for the MRS for this utility function. Show how this can be interpreted as the ratio of the marginal utilities for X and Y.

U(X,Y) = (XY)1/2. Taking the total differential

dU = UXdX+UYdY = .5(Y/X).5dX +.5(X/Y).5dY = 0

Solving the middle equality

dY/dX = -UX/UY

Solving the rightmost expression

. dY/dX = -Y/X so the MRS = Y/X (the opposite of dY/dX)

d. Consider a logarithmic transformation of this utility function

U’ = logU

Where log is the logarithmic function to base 10. Show that for this transformation the U’=1 indifference curve has the same properties as the U=10 curve calculated in parts (a) and (b). What is the general expression for the MRS of this transformed utility function?

U’ = log (XY)1/2 = .5logX + .5logY

Plotting ordered pairs when U’=1 yields

|X |Y |U |logX |logY |U' |MRS |

|5 |20 |100 |0.69897 |1.30103 |1 |4 |

|10 |10 |100 |1 |1 |1 |1 |

|15 |6.666667 |100 |1.176091 |0.823909 |1 |.44 |

|20 |5 |100 |1.30103 |0.69897 |1 |.25 |

Obviously indifference curves are the same for each utility function.

One can totally differentiate U’ to obtain the same general expression for the MRS as before:

dU’ = (.5/X)dX + (.5/Y)dY = 0

Solving dY/dX = -Y/X so the MRS = Y/X

3.4 For each of the following expressions, state the formal assumption that is being made about the individual’s utility function.

a. It (margarine) is just as good as the high-price spread (butter).

MRSmb = 1, where m = margarine and b = butter.

b. Peanut butter and jelly go together like a horse and carriage

Peanut butter and jelly are perfect complements. That is

U(peanut butter, jelly) = min{peanut butter, jelly}

Where the terms “peanut butter” and “jelly” refer to servings of each product.

c. Things go better with Coke.

Coca Cola is a complement for all goods. That is, for any good x

Ux, coca cola>0

d. Popcorn is addictive – the more you eat, the more you want.

Popcorn consumption exhibits increasing marginal utility, e.g.,

Upopcorn >0.

e. Mosquitoes ruin a nice day at the beach.

Mosquito avoidance and a (mosquito free) day at the beach are perfect substitutes. Let the incremental utility of the day at the beach be U(beach)>0. Then the incremental utility of a day at the beach with mosquitoes is U(beach, mosquitoes) < 0. In other words, the marginal utility of a day at the beach is less than or equal to the marginal disutility of mosquitoes.

f. A day without wine is like a day without sunshine. The marginal (more precisely the incremental) utility of a “wine” just equals the marginal (incremental) utility of sunshine in a day.

g. It takes two to tango. “tango” dancing and a partner are perfect complements in consumption. U(tango, partner) = min(tango, partner)

3.5 Graph a typical indifference curve for the following utility functions and determine whether they have convex indifference curves (that is, whether they obey the assumption of a diminishing MRS)

a. U = 3X + Y

Here the MRS = -dY/dX = 3. The MRS is a constant, and does not exhibit diminishing MRS.

b. U = (XY).5

|X |Y |U |MRS |

|5 |20 |10 |4 |

|10 |10 |10 |1 |

|15 |6.6666667 |10 |0.444444 |

|20 |5 |10 |0.25 |

Here MRS is –dY/dX = X/Y. As seen in the rightmost column of the above table, this does exhibit diminishing MRS

c. U= (X2 + Y2).5

Suppose we confine attention to constant increments of X and a utility level of 28.28.

|X |Y |U |MRS |

|20 |20 |28.284271 |1 |

|15 |23.976 |28.281594 |0.625626 |

|10 |26.455 |28.28192 |0.378 |

|5 |27.836 |28.281494 |0.179624 |

Here the utility function is obviously concave, implying an increasing MRS. More formally,

dU = X(X2 + Y2)-.5dX+ Y(X2 + Y2)-.5dY =0 implies

dY/dX = - X/Y.

Values are shown in the rightmost column of the above table. Notice that the MRS moves directly with X (Constant increments of X require giving up increasing increments of Y)

Notice: What does this imply about the preferred consumption bundle? It implies that for a given budget constraint, utility will be maximized at one corner or the other. For example, an agent may want to select either a collection of modern black and silver furniture for a room, or 18th century Jaocbian antiques, depending on the relative price. But the consumer may be worse of with a combination of the two.

d. U= (X2 - Y2).5

Plotting some points

|X |Y |U |MRS |

|20 |17.315 |10.009534 |-1.15507 |

|15 |11.175 |10.005967 |-1.34228 |

|12 |6.63 |10.002155 |-1.80995 |

|11 |4.55 |10.014864 |-2.41758 |

|10 |0 |10 |#DIV/0! |

| | | | |

Graphically

Here, notice the Y is a “bad.” Thus, the slope of the MRS is negative. More formally,

dU = X(X2-Y2)-.5dX –Y(X2-Y2)-.5dY = 0

implies

dY/dX = X/Y

Given a budget constraint, with positive prices for both, the consumer would maximize utility by purchasing only X. On the other hand, if we framed the problem in terms of Y removal, then U = (X2+Y2).5 as we established in the part c, the MRS for such a problem–X/Y exhibits an increasing MRS. So even were Y presented as a “good” the consumer would consume either X or ‘not’ Y.

e. U = X2/3Y1/3

|X |Y |U |MRS |

|20 |2.5 |10 |0.25 |

|15 |4.45 |10.00417 |0.593333 |

|10 |10 |10 |2 |

|5 |40 |10 |16 |

This is another variant of a Cobb-Douglas function. The function does exhibit diminishing MRS. Formally,

dU = (2/3)X-1/3Y1/3)dX

+ (1/3) X2/3Y-1/3)dY =0

implies

dY/dX = -2Y/X.

Values are shown in the rightmost column of the above table.

f. U = log X + log Y. We analyzed this function in problem 3.2(d). Looking the table shown below, it is obvious that the MRS for this function is the same as for 3.5(b).

|X |Y |U |MRS |logX |logY |U' |

|20 |5 |10 |0.5 |1.30103 |0.69897 |1 |

|15 |6.6666667 |10 |0.888889 |1.1760913 |0.8239087 |1 |

|10 |10 |10 |2 |1 |1 |1 |

|5 |20 |10 |8 |0.69897 |1.30103 |1 |

Formally, dU = dX/X + dY/Y = 0

Solving dY/dX = -Y/X

3.7. Consider the following utility functions. Show that each of these has a diminishing MRS, but that they exhibit constant, increasing and decreasing marginal utility, respectively. What can you conclude?

a. U(X,Y) = XY

MRS: dU = YdX + XdY = 0

Implies that dY/dX = -Y/X. This is diminishing MRS.

Not consider marginal utility.

U1 = Y, U2 = X. These are both positive for any positive combination of Y and X. The second order conditions U11 = 0, U22 = 0 suggest that they have constant marginal utility for each good.

b. U(X,Y) = X2Y2

MRS: dU = 2XY2dX + 2YX2dY = 0

Implies that dY/dX = -Y/X. This is the same as above, diminishing.

Utility. In the above function U11 =2Y2, U22 = 2X2. These are both positive, so the function exhibits increasing marginal utility. (More generally, with

U11 >0, U22 >0 and U12 = 4XY, we have U11 U22 - U122 = 4X2Y2 -16X2Y2 ................
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