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Lionel computed the average rate of change in the depth of a pool over a two-week interval to be zero. Which statement must be true?

A. The pool must have been empty for the entire interval.
B. The pool must have been the same depth at the start of the interval as it was at the end of the interval.
C. The pool must have been deeper at the end of the interval than it was at the start of the interval.
D. The pool must have been more shallow at the end of the interval than it was at the start of the interval.

Sagot :

Let's address the problem step by step to determine which statement must be true given that the average rate of change in the depth of the pool over the two-week interval is zero.

1. Understanding Average Rate of Change:
- The average rate of change of a function over an interval [tex]\([a, b]\)[/tex] is the change in the value of the function divided by the length of the interval. Mathematically, it is given by:
[tex]\[ \text{Average rate of change} = \frac{f(b) - f(a)}{b - a} \][/tex]
- If this rate of change is zero, it means there is no net change in the function's value over the interval [tex]\( [a, b] \)[/tex].

2. Applying to Pool Depth:
- Let [tex]\(d(t)\)[/tex] be the depth of the pool at time [tex]\( t \)[/tex]. Over the interval from time [tex]\( t_1 \)[/tex] (start) to [tex]\( t_2 \)[/tex] (end), the depth did not change.
- The average rate of change is given by:
[tex]\[ \frac{d(t_2) - d(t_1)}{t_2 - t_1} \][/tex]
- Given that the average rate of change is zero, we have:
[tex]\[ \frac{d(t_2) - d(t_1)}{t_2 - t_1} = 0 \][/tex]
- This implies:
[tex]\[ d(t_2) - d(t_1) = 0 \][/tex]
[tex]\[ \text{Therefore, } d(t_2) = d(t_1) \][/tex]

3. Interpreting the Given Choices:
- Option 1: "The pool must have been empty for the entire interval."
- This statement is not necessarily true. The pool's depth could have been any constant value, not necessarily zero, as long as it remained unchanged.
- Option 2: "The pool must have been the same depth at the start of the interval as it was at the end of the interval."
- This statement accurately describes the condition we determined mathematically: [tex]\( d(t_2) = d(t_1) \)[/tex].
- Option 3: "The pool must have been deeper at the end of the interval than it was at the start of the interval."
- This statement is false since we determined the depth did not change.
- Option 4: "The pool must have been more shallow at the end of the interval than it was at the start of the interval."
- Similarly, this statement is false for the same reason.

4. Conclusion:
- The correct statement is:
[tex]\[ \text{The pool must have been the same depth at the start of the interval as it was at the end of the interval.} \][/tex]

So, the true statement is:
The pool must have been the same depth at the start of the interval as it was at the end of the interval.