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The speed of an elevator (in feet per second) is modeled by the function [tex]f(x) = 1.6875x[/tex], where [tex]x[/tex] is time in seconds. Estimate the average rate of change between 3.9 seconds and 8.2 seconds. Round the final answer to two decimal places.

A. about 6.75 feet/second
B. about 1.69 feet/second
C. about 4.00 feet/second
D. about 0.59 feet/second

Answer :

To find the average rate of change of the elevator's speed between 3.9 seconds and 8.2 seconds, we'll use the given function for speed, [tex]\( f(x) = 1.6875x \)[/tex].

Here are the steps to calculate this:

1. Identify the function: The speed of the elevator is given by the function [tex]\( f(x) = 1.6875x \)[/tex], where [tex]\( x \)[/tex] is the time in seconds.

2. Evaluate the function at the given times:

- Find the speed at the start time, [tex]\( x = 3.9 \)[/tex] seconds:
[tex]\( f(3.9) = 1.6875 \times 3.9 = 6.58125 \)[/tex] feet/second.

- Find the speed at the end time, [tex]\( x = 8.2 \)[/tex] seconds:
[tex]\( f(8.2) = 1.6875 \times 8.2 = 13.8375 \)[/tex] feet/second.

3. Calculate the average rate of change:
The average rate of change of the speed between 3.9 seconds and 8.2 seconds is given by the change in speed divided by the change in time.

[tex]\[
\text{Average rate of change} = \frac{f(8.2) - f(3.9)}{8.2 - 3.9}
\][/tex]

- Calculate the change in speed:
[tex]\( 13.8375 - 6.58125 = 7.25625 \)[/tex] feet/second.

- Calculate the change in time:
[tex]\( 8.2 - 3.9 = 4.3 \)[/tex] seconds.

- Determine the average rate of change:
[tex]\[
\text{Average rate of change} = \frac{7.25625}{4.3} \approx 1.69
\][/tex]

Hence, the average rate of change of the elevator's speed between 3.9 seconds and 8.2 seconds is approximately [tex]\( 1.69 \)[/tex] feet per second. This matches with the choice "about 1.69 feet/second".

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