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

Answers

Answer 1
Answer: 52 inches - 12 inches = 40 inches
amplitude:  a = 40 inches / 2 = 20 

f(x)=20cos(bx)+c
the value of c is 32... since the centre of the has been moved up 32 units

the minimum amplitude =  32 - 20 = 12
the maximum amplitude = 32 + 20 = 52

f(x)=20cos(bx)+32
if the curve takes 6 1/4 hours from low to high tides (9:15 am to 3:30 pm)  then it will take 12 1/2 hours to complete a full cycle.

adjust the period by converting 12 1/2 hours to an angle measure.

360
°/12 = 30° 
30° / 12 = 15°

12 1/2 = 360° + 15° = 375°

f(x) = 20 cos(375°) + 32
f(x) = 20 * 0.97 + 32
f(x) = 19.4 + 32
f(x) = 51.4
Answer 2
Answer:

The cosine function that models the variation is f(x) = 51.4

Calculations and Parameters:

To find the inches, we would subtract the value of 12 from 52 which would give us 40 inches.

The amplitude is 40/2

= 20

Hence,

f(x)=20cos(bx)+c

  • c= 32
  • the minimum amplitude = 12
  • the maximum amplitude = 52

f(x)=20cos(bx)+32

if the curve takes 6 1/4 hours from low to high tides (9:15 am to 3:30 pm)  then it will take 12 1/2 hours to complete a full cycle.

We make adjustments and convert

  • 360°/12 = 30°
  • 30° / 12 = 15°
  • 12 1/2 = 360° + 15° = 375°

f(x) = 20 cos(375°) + 32

f(x) = 20 * 0.97 + 32

f(x) = 19.4 + 32

f(x) = 51.4

Read more about cosine functions here:

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I'm sure you know how to find the slope of a line if you know 2 points
on it ...  The slope is ...

       (difference in y-values) / (difference in x-values) .

Right ?

OK.  Each line in these tables is a point on the graph of the table.
Pick two points from the table, and you can get the slope of the line
from them.

The first table (Plan-A) has these points in it:

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2 . . . . . $20.30
3 . . . . . $20.45
4 . . . . . $20.60

You want to find the equation of the line with these 4 points on it.
That means you need to find the slope and the intercept of the line.
Take any two points straight from the table.  I'll use the first two:

1 . . . . . $20.15
2 . . . . . $20.30

There you have two points on the line:  (1, 20.15) and (2, 20.30) .
The slope is  (difference in 'y') / (difference in 'x').

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Difference in 'x' = (2 - 1) = 1

Slope of the line =  (0.15) / 1  =  0.15
(What that really means is 15 cents per minute on Plan-A.)

Now you know that the equation of the line is [ y = 0.15x + intercept ].

Can you think of a way to find the intercept ?
Remember ... every point in the table is on the line.
So why not just take one of these points in the table, put it into the part of the equation that you already have, and watch the intercept fall out ?
You could use any point at all from the table.
I'll use the 3rd one.

                                                         y = 0.15x + intercept

                                                  20.45 = 0.15(3) + intercept

                                                  20.45 = 0.45 + intercept

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===============================

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Difference in x-values = (10 - 5) = 5

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Intercept = 5.00

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======================================

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