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Math 150A Writing Project
In order to satisfy the GE university writing requirements for the course, you are to
complete writing project. You must complete one of the following problems and
produce a two-page write-up of the problem (in your own words), the full solution (with
justification of each step of the process and any conclusions drawn), and interpretation of
the results. Any variables must clearly be defined. Give answers in fractional form (not
decimal), if necessary.
Problem 1 – Filling Swimming Pools:
Spring in Southern California has finally arrived. You are looking forward to filling your
swimming pool, which has a length of 10 meters and ends with trapezoidal shape. The
trapezoids on the ends have a bottom edge with length 6 meters, a top edge with length
12 meters, and a height of 2 meters. Your hose can produce 3 cubic meters of water per
hour. You begin to fill your pool. When the height of the water reaches 0.1 m, you
realize this will take a long time. Calculate the rate the height of the water is increasing
when it is 0.1 m high. Use this value to estimate how long it will take to fill the pool. Is
this an overestimate or underestimate? Explain your reasoning.
Problem 2 – Party Games:
Setting up for a birthday party, you decide to play a game with party hats. The hats are
in the shape of a cone with height 10 inches and radius 6 inches. The hats are placed on
a table with the point towards the ceiling. The goal of the game is guess the correct hat
with a prize hidden underneath it. The prize is contained in a cylindrical cup. You need
to determine the dimensions of the cylindrical cup (radius and height) that can hold the
most and still be hidden under a party hat. What is the most that this optimal cup can
hold? Given this information, what would you include as the prize?
Requirements:
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At least 2 FULL pages, typed, double-spaced, 12 point font, 1 inch margins.
No grammatical, punctuation, or spelling errors.
Your name and title should be included in the header of the document and not the body.
Use the sample template.
Figures may be included on a separate page at the end, which will not be counted
towards the 2 full pages.
Academic Dishonesty
I will pursue these cases. Do not plagiarize. If you work with another person, be sure to
produce the write-up independently.
Due Thursday, June 28, 2018 at 8:00 A.M.
Math 150A
Ladder Around a Corner
Laura Smith, Ph.D.
We are to carry a stepladder down a hallway that is 9 feet wide. Unfortunately, at
the end of the hallway there is a corner, where we must make a right-angled turn into a
hallway that is 6 feet wide. Our goal is to determine the maximum length of a ladder that
could be carried horizontally around this corner. In order to solve this problem, we must
consider a ladder that touches the exterior wall in our hallway, the exterior wall in the
other hallway, and the interior corner. Any ladder that cannot touch all three will be short
enough to pass. Thus, the problem has turned into finding the shortest ladder that will
touch all three.
Let L be the length of a ladder that touches the exterior wall in the 9-foot hallway at
point A, the exterior wall in the 6-foot hallway at point B, and the interior corner at point C
(see Figure 1). Extending the walls of the hallways on a diagram, we let D be the point
where the 6-foot hallway extension intersects the wall, and let E be the point where the 9foot hallway extension intersects the other wall (see Figure 1). Let x be the length of line
segment AC, and let y be the length of line segment CB. We notice that the angle between
line segments DC and AD is the same as the angle between line segments EB and BC.
Using the definitions of trigonometric functions, we can write the length of the
ladder L as a function of this angle ,
𝐿(𝜃) = 𝑥 + 𝑦 = 9 ∙ csc(𝜃) + 6 ∙ sec(𝜃).
𝜋
The domain of this function is 0 < 𝜃 < 2 . To find the minimal L, we find our absolute
minimum using the first derivative test for absolute extreme values. Thus, we need to find
the critical numbers by taking the derivative. We have
𝐿′ (𝜃) = −9 ∙ csc(𝜃) ∙ cot(𝜃) + 6 ∙ sec(𝜃) ∙ tan(𝜃).
To find the critical numbers, we set our derivative equal to zero. This gives us
0=
−9cos(𝜃)
sin2 (𝜃)
+
6sin(𝜃)
cos2 (𝜃)
=
−9 cos3(𝜃)+6sin3 (𝜃)
sin2 (𝜃)cos2 (𝜃)
,
which is zero when the numerator is equal to zero. Hence, −9 cos 3 (𝜃) + 6sin3 (𝜃) = 0, or
3
equivalently, 2 = 𝑡𝑎𝑛3 (𝜃). Thus, one critical number is
3 1/3
𝜃 = 𝑡𝑎𝑛−1 ((2)
).
We then search for when the derivative is undefined, which occurs when either sine or
cosine is equal to zero on the domain. However, there are no such values on our domain.
We now examine where L’() is positive and negative on our domain. We have that
3 1/3
our derivative is negative for 0