Question 1194135

The function will be he water level varies from 12 inches at low tide to 52 inches at high tide. Low tide occurs at 9:15 am. High tide occurs at 3:30 pm. What is a cosine function that models the variation in inches above and below the water level as a function of time and hours since 9:15 am?

Please tell me what should I put as answer for the a cosine function. From chegg I got the answer as f(x) = 20 (x -0.99 ) + 32. Is that correct? 
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It's obviously not correct as it's not a cosine function.
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The peak-to-peak is 52-12 = 40 inches --> amplitude = 20 inches
The average level is (12+52)/2 = 32
The time at 32" is (1530 + 0915)/2 = 1222.5 (12:22.5) PM
The period = 2*(1530-0915) = 12.5 hours
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The function will be {{{20*cos((t*(2pi/12.5))+pi) + 32}}}