AP
®
CALCULUS AB
2008 SCORING COMMENTARY
Question 2
Overview
This problem presented students with a table of data indicating the number of people
Lt
in line at a concert
ticket office, sampled at seven times t during the 9 hours that tickets were being sold. (The question stated that
Lt
was twice differentiable.) Part (a) asked for an estimate for the rate of change of the number of people in line
at a time that fell between the times sampled in the table. Students were to use data from the table to calculate an
average rate of change to approximate this value. Part (b) asked for an estimate of the average number of people
waiting in line during the first 4 hours and specified the use of a trapezoidal sum. Students needed to recognize
that the computation of an average value involves a definite integral, approximate this integral with a trapezoidal
sum, and then divide this total accumulation of people hours by 4 hours to obtain the average. Part (c) asked for
the minimum number of solutions guaranteed for
0Lt
during the 9 hours. Students were expected to
recognize that a change in direction (increasing/decreasing) for a twice-differentiable function forces a value of 0
for its derivative. Part (d) provided the function tickets per hour as a model of the rate at which
tickets were sold during the 9 hours and asked students to find the number of tickets sold in the first 3 hours, to
the nearest whole number, using this model. Students needed to recognize that total tickets sold could be
determined by a definite integral of the rate
()
/2
550
t
rt te
−
=
rt
at which tickets were sold.
Sample: 2A
Score: 9
The student earned all 9 points. In part (c) the student might have given a more complete justification for the
existence of a local maximum on the interval
1, 4 .
It would have been better if the student had used the word
“graph” rather than “line” in the first paragraph. The response is more complete and uses better terminology in the
second and third paragraphs, and the student gives the correct answer.
Sample: 2B
Score: 6
The student earned 6 points: 2 points in part (a), no points in part (b), 2 points in part (c), and 2 points in part (d). The
student earned both points in part (a) with a correct estimate and correct units. In part (b) the student’s expression
reflects subdivisions of
[
of equal length. The student did not earn any points. In part (c) the student earned the
first point by considering
]
0, 4
Lt
changing from increasing to decreasing. (The student goes on to consider
Lt
changing from decreasing to increasing, but this was not necessary to earn the first point.) The student did not earn
the second point since it is not necessarily true that
Lt
changes from increasing to decreasing on the interval
The student earned the third point with the correct answer of 3. The student earned both points in part (d).
[]
3, 4 .
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