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80 CHAPTER 3 CONSUMER PREFERENCES AND THE CONCEPT OF UTILITY

U(y), total utility of hamburgers R A S (a) B N C K U = ( y)

M
L

0 2 7 9
y, weekly consumption of hamburgers

(b)
MU y , marginal utility of hamburgers

FIGURE 3.3 Marginal Utility May Be Negative A'
The utility curve U(y) is shown in panel (a), and the
corresponding marginal utility curve is illustrated in
panel (b). The slope of the utility curve in the top panel
is positive at A; thus, the marginal utility is positive, as
indicated at point A in panel (b). At point B the slope B'
of the utility curve is zero, meaning that the marginal
utility is zero, as shown at point B . At point C the 0 2 7 C' 9
slope of the utility function is negative; therefore, the MU y
marginal utility is negative, as indicated at point C . y, weekly consumption of hamburgers

is positive, even though it may be diminishing with each additional hamburger she
eats. But presumably at some point she will find that an additional hamburger will
bring her no more satisfaction. For example, she might find that the marginal utility
of the seventh hamburger per week is zero, and the marginal utility of the eighth or
ninth hamburgers might even be negative.
Figure 3.3 depicts the total and marginal utility curves for this case. Initially (for
values of y 7 hamburgers), total utility rises as consumption increases, and the slope
of the utility curve is positive (e.g., note that the segment RS, which is tangent to the
utility curve at point A when Sarah is purchasing her second hamburger, has a posi-
tive slope); thus, the marginal utility is positive (as depicted at point A ). However, the
marginal utility is diminishing as consumption increases, and at a consumption level
of seven hamburgers, Sarah has purchased so much of the good that the marginal util-
ity is zero (point B ). Since the marginal utility is zero, the slope of the total utility
curve is zero. (The segment MN, which is tangent to the utility curve at point B, has a
slope of zero.) If Sarah were to buy more than seven hamburgers, her total satisfaction