AP
®
CALCULUS AB
2010 SCORING COMMENTARY
© 2010 The College Board.
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Question 2
Overview
This problem involved a zoo’s contest to name a baby elephant. Students were presented with a table of values
indicating the number of entries
E
(
t
)
,
measured in hundreds, received in a special box and recorded at various
times t during an eight-hour period. Part (a) asked for an estimate of the rate of deposit of entries into the box at
time 6.t = Students needed to recognize this rate to be the derivative value
(
6E
′
)
. Since t = 6 falls between the
time values specified in the table, students needed to calculate the average rate of change of E across the smallest
time subinterval from the table that brackets
t = 6. Part (b) asked for an approximation to
()
8
0
1
8
E tdt
using a
trapezoidal sum and the subintervals of [0, 8] indicated by the data in the table. Students were further asked to
interpret this expression in context, with the expectation that they would recognize that it gives the average
number of hundreds of entries in the box during the eight-hour period. In part (c) a function P was supplied that
models the rate at which entries from the box were processed, by the hundred, during a four-hour period
(
8≤≤t
)
12
that began after all entries had been received. This part asked for the number of entries that remained
to be processed after the four hours. Students needed to recognize that the number of entries processed is given by
()
12
8
Pt d
t, so that the number remaining to be processed, in hundreds of entries, is given by the difference
between the total number of entries in the box,
E
(
8
)
,
as given by the table, and the value of this integral. Part (d)
cited the model P
(
t
)
introduced in the previous part and asked for the time at which the entries were being
processed most quickly. Students should have recognized this as asking for the time corresponding to the
maximum value of
P
(
t
)
on the interval 8 12t≤≤ and applied a standard process for optimization on a closed
interval.
Sample: 2A
Score: 9
The student earned all 9 points.
Sample: 2B
Score: 6
The student earned 6 points: 1 point in part (a), 2 points in part (b), 2 points in part (c), and 1 point in part (d). In
part (a) the student sets up a correct difference quotient based on the values in the table and correctly evaluates for
the numerical answer. In part (b) the student sets up a correct trapezoidal sum and evaluates it based on the data in
the table to obtain a correct approximation. The student did not earn the third point in part (b) because the meaning
given does not address the time interval over which the average was computed. In part (c) the student earned both
points. The first point was earned for correctly providing the definite integral that represents the number of
hundreds of entries processed between 8
P.M. and midnight. The second point was earned for subtracting that
value from the initial condition of 23 hundred entries in the box at 8
P.M. to obtain the answer. In part (d) the
student earned the first point for setting
()
0.Pt
′
= The student does not consider the endpoints as candidates, so
no additional points were earned.