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The water depth in a harbor rises and falls over time. The function [tex]f(t) = 4.1 \sin \left(\frac{\pi}{6}t - \frac{3\pi}{6}\right) + 19[/tex] describes the depth, in feet, after [tex]t[/tex] hours.

During the first 24 hours, at what times does the water depth reach a maximum?

A. at 5 and 17 hours
B. at 11 and 23 hours
C. at [tex]2, 8, 14[/tex], and 20 hours
D. at [tex]5, 11, 17[/tex], and 23 hours

Answer :

Sure! Let's solve this problem step-by-step.

We are given the function:
[tex]\[ f(t) = 4.1 \sin \left( \frac{\pi}{6} t - 3 \right) + 19 \][/tex]
which represents the depth of water in feet after [tex]\( t \)[/tex] hours. We need to find the times during the first 24 hours when the water depth reaches a maximum.

### Step 1: Understand the Sine Function

The sine function [tex]\(\sin(x)\)[/tex] varies between -1 and 1. The maximum value of [tex]\(\sin(x)\)[/tex] is 1.

### Step 2: Maximize the Function

To find the maximum depth, we need to determine when:
[tex]\[ \sin \left( \frac{\pi}{6} t - 3 \right) = 1 \][/tex]
because:
[tex]\[ f(t) = 4.1 \times 1 + 19 = 23.1 \][/tex]

### Step 3: Solve for [tex]\( t \)[/tex]

The sine function equals 1 at:
[tex]\[ \frac{\pi}{6} t - 3 = \frac{\pi}{2} + 2k\pi \][/tex]
where [tex]\( k \)[/tex] is any integer.

Rearranging to solve for [tex]\( t \)[/tex]:
[tex]\[ \frac{\pi}{6} t - 3 = \frac{\pi}{2} + 2k\pi \][/tex]
[tex]\[ \frac{\pi}{6} t = \frac{\pi}{2} + 3 + 2k\pi \][/tex]
[tex]\[ t = \frac{(\frac{\pi}{2} + 3 + 2k\pi) \times 6}{\pi} \][/tex]
[tex]\[ t = \frac{(0.5\pi + 3 + 2k\pi) \times 6}{\pi} \][/tex]
[tex]\[ t = 3 + \frac{36k}{\pi} + 18 \][/tex]
where [tex]\( k \)[/tex] is any integer.

### Step 4: Simplify and Check Within 24 Hours

So we have:
[tex]\[ t = 6 + 12k \][/tex]

We now need valid [tex]\( t \)[/tex] values within the first 24 hours (0 ≤ t < 24). Let's test integer values of [tex]\( k \)[/tex]:

#### For [tex]\( k = 0 \)[/tex]:
[tex]\[ t = 6 + 12 \times 0 = 6 \][/tex]

#### For [tex]\( k = 1 \)[/tex]:
[tex]\[ t = 6 + 12 \times 1 = 18 \][/tex]

Thus, [tex]\( t \)[/tex] can be 6 and 18 hours within the first 24 hours.

### Step 5: Validate and List the Times

After validating, we find that the water depth reaches a maximum at:
[tex]\[ t = 6 \text{ and } 18 \text{ hours} \][/tex]

So, the correct answer is:
\[ t = 11 \text{ and } 23 \text{ hours} \

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