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Answer:
(d) Amplitude = 8 feet; period = 18 hours; midline: y = 12
Explanation:
The amplitude is half the difference between the low and high, so is ...
(20 -4)/2 = 16/2 = 8 . . . . amplitude in feet
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The period is the time it takes for one full cycle. The problem statement tells you one full cycle takes 18 hours, so 18 hours is the period.
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The midline is half the sum of the low and high, so is ...
(20 +4)/2 = 24/2 = 12 . . . . midline in feet
These values are matched by the last choice:
amplitude 8, period 18, midline 12.