Math 181 Miniproject 5: Hours of Daylight.md
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tags: MATH 181
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Math 181 Miniproject 5: Hours of Daylight
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**Overview:** This miniproject will apply what you've learned about derivatives so far, especially the Chain Rule, to analyze the change the hours of daylight.

**Prerequisites:** The computational methods of Sections 2.1--2.5 of *Active Calculus*, especially Section 2.5 (The Chain Rule).

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:::info
The number of hours of daylight in Las Vegas on the $x$-th day of the year ($x=1$ for Jan 1) is given by the function together with a best fit curve from Desmos.}[^first]
[^first]: The model comes from some data at http://www.timeanddate.com/sun/usa/las-vegas?

\\[
D(x)=12.1-2.4\cos \left(\frac{2\pi \left(x+10\right)}{365}\right).
\\]
(1) Plot a graph of the function $D(x)$. Be sure to follow the guidelines for formatting graphs from the specifications page for miniprojects.
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(1)

:::info
(2) According to this model how many hours of daylight will there be on July 19 (day 200)?
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(2)
14.236 Hours/daylight

:::info
(3) Go to http://www.timeanddate.com/sun/usa/las-vegas? and look up the actual number of hours of daylight for July 19 of this year. By how many minutes is the model's prediction off of the actual number of minutes of daylight?
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(3)
According to the website the actual numbers of hours of daylight is 14:18 hrs. The models was about 0.6 min away.
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(4) Compute $D'(x)$. Show all work.
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(4)
$D(x)=12.1-2.4\cos \left(\frac{2\pi \left(x+10\right)}{365}\right).$

$D'(x)=12.1-2.4\cos \left(\frac{2\pi \left(x+10\right)}{365}\right).$

$2.4\sin\left(\frac{2\pi\left(x+10\right)}{365}\right)\cdot\frac{\frac{d}{dx}\left[2\pi x+20\pi\right]\cdot365+\left(2\pi x+20\pi\right)\cdot\frac{d}{dx}\left[365\right]}{\left(365\right)^{2}}$

$=2.4\sin\left(\frac{2\pi\left(x+10\right)}{365}\right)\ \frac{\left(2\pi\cdot365+\left(2\pi x+20\pi\right)0\right)}{\left(365\right)^{2}}$

$=2.4\sin\left(\frac{2\pi x+20\pi}{365}\right)\ \frac{\left(730\pi\right)}{\left(365\right)^{2}}$

:::info
(5) Find the rate at which the number of hours of daylight are changing on July 19. Give your answer in minutes/day and interpret the results.
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(5)
$D'\left(x\right)=2.4sin\left(\frac{2\pi\left(x+10\right)}{365}\right)\cdot\left(\frac{730\pi}{365^{2}}\right)$
$D'\left(200\right)=2.4sin\left(\frac{2\pi\left(200+10\right)}{365}\right)\cdot\left(\frac{730\pi}{365^{2}}\right)$
$D'\left(200\right)=-0.0188353725245\ \frac{hr}{day}$
$-\frac{0.0188353725245}{1\ day}\cdot\frac{60\min}{1hr}=-\frac{1.13012235147\min}{1\ day}$

The rate in which July 19 is changing by -1.13012235147 min/day. So the days are decreasing.

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(6) Note that near the center of the year the day will reach its maximum length when the slope of $D(x)$ is zero. Find the day of the year that will be longest by setting $D'(x)=0$ and solving.
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(6)
$D'\left(x\right)=2.4\sin\left(\frac{2\pi\left(x+10\right)}{365}\right)\cdot\left(\frac{730\pi}{365^{2}}\right)$

$0=2.4\sin\left(\frac{2\pi x+20\pi}{365}\right)$

$0=\sin\left(\frac{2\pi x+20\pi}{365}\right)$

$\sin^{-1}\left(0\right)=\frac{2\pi x+20\pi}{365}$

$365\sin^{-1}\left(0\right)=2\pi x+20\pi$

$365\sin^{-1}\left(0\right)-20\pi=2\pi x$

$\frac{365\sin^{-1}\left(0\right)-20\pi}{2\pi}=\pi$

$-10=x$
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(7) Write an explanation of how you could find the day of the year when the number of hours of daylight is increasing most rapidly.
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(7)
I believe that you can find it by looking at desmos and by looking at the function where the graph appear to have a steepness positive slope.

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