This isn't rocket science, it's rocket math!

Geometry Level 2

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In this above rocket picture,

  • The top black line serves as the height of the cone portion of the rocket and the bottom black line serves as the height of the "lamp shade" portion of the rocket. Both black lines have a length of 5 inches.
  • The red line serves as the height of the cylindrical portion of the rocket and has a length of 10 inches.
  • The green line serves as the radius of the cone portion and cylindrical portion. The green line also serves as the smaller radius of the lampshade portion. The length of the green line is 1 inch.
  • The blue line serves as the larger radius of the lampshade portion, and has a length of 2 inches.

    If the surface area of the rocket (in square inches) equals S A SA , find S A \left \lfloor SA \right \rfloor .

Note: This "rocket" has a sealed base. Please include the base in your answer.


The answer is 139.

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4 solutions

Relevant wiki: Surface Area - Problem Solving

NOTE: The figure above is not drawn to scale.

Let A A be the surface area of the top part, B B be the surface area of the middle part and C C be the surface area of the bottom part. Then,

A = 1 2 c L = 1 2 ( π ) ( 2 ) 5 2 + 1 2 = π 26 A=\dfrac{1}{2}cL=\dfrac{1}{2}(\pi)(2)\sqrt{5^2+1^2}=\pi \sqrt{26}

B = c h = 2 π ( 10 ) = 20 π B=ch=2\pi(10)=20 \pi

C = 1 2 ( c 1 + c 2 ) L + π r 2 = 1 2 π ( 2 + 4 ) 5 2 + 1 2 + π ( 2 2 ) = 1 2 π ( 6 ) 26 + 4 π = 3 π 26 + 4 π C=\dfrac{1}{2}(c_1+c_2)L + \pi r^2=\dfrac{1}{2} \pi(2+4)\sqrt{5^2+1^2} + \pi (2^2)=\dfrac{1}{2}\pi(6)\sqrt{26}+4\pi=3\pi\sqrt{26}+4\pi

Finally,

S A = A + B + C = π 26 + 20 π + 3 π 26 + 4 π SA=A+B+C=\pi\sqrt{26}+20\pi+3\pi\sqrt{26}+4\pi \approx 139 \color{#D61F06}\boxed{139} a n s w e r \boxed{answer}

Definitions:

c c = circumference of the circle

L L = slant height

r r = radius of the circle

h h = height

c 1 c_1 = upper circumference

c 2 c_2 = lower circumference

Akshat Sharda
Feb 17, 2016

CSA of the cone portion = π r l = π 1 h 2 + r 2 = π 26 =\pi r l = \pi \cdot 1 \cdot \sqrt{h^2+r^2}=\pi \sqrt{26}

CSA of cylinder portion = 2 π r h = 2 π 1 10 = 20 π =2\pi r h = 2\pi \cdot 1 \cdot 10=20\pi

CSA of frustum portion = π ( r 1 + r 2 ) h 2 + ( r 1 r 2 ) 2 + π r 2 2 = 3 π 26 + 4 π =\pi (r_{1}+r_{2})\sqrt{h^2+(r_{1}-r_{2})^2}+\pi r_{2}^2=3\pi \sqrt{26}+4\pi

The Total SA, = 4 π 26 + 24 π 139.47 139.47 = 139 =4\pi \sqrt{26}+24\pi\sim 139.47 \\ \Rightarrow \lfloor 139.47 \rfloor = \boxed{139}

The point of this problem was that you didn't need the frustrum portion. You just needed to combine the top cone with the bottom frustrum to get a bigger cone. The ratios add up.

Hobart Pao - 5 years, 3 months ago

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Aah !! I can see it now !

Akshat Sharda - 5 years, 3 months ago

We don't even need the radius of the cone thingy on the top, we only need similarities to prove that it is half of the radius of the lampshade thingy at the bottom.

Manuel Kahayon - 5 years, 3 months ago

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yup that's what i meant

Hobart Pao - 5 years, 3 months ago

Dont you think that was a good practice for Boards Summatives..!! Ahhh....

Anshuman Singh Bais - 5 years, 3 months ago

But did you include the base? It is a rocket and has an open base.

Rishik Jain - 5 years, 3 months ago

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I meant to include the base. I'll clarify.

Hobart Pao - 5 years, 3 months ago
David Orrell
Oct 12, 2016

The two conical bits (frustum at the bottom and cone at the top) have the same height, and the radius of the top cone is half the radius of the bottom of the frustum. Proportions therefore allow us to put these two pieces together to produce a cone of height 2*5 = 10. The slant length, l l , of this cone is 1 0 2 + 2 2 \sqrt{10^{2}+2^{2}} , by Pythagoras' Theorem. Since the curved surface area of a cone is π r l \pi rl , we find that it is 2 π 1 0 2 + 2 2 2\pi\sqrt{10^{2}+2^{2}} .

Next, the area of a circle is π r 2 \pi r^{2} , so the area of the base is 4 π 4\pi .

Finally the curved surface area of a cylinder is π r 2 h \pi r^{2}h . This is 20 π 20\pi here.

The total is 2 π 104 + 4 π + 20 π 2\pi \sqrt{104} + 4\pi + 20\pi .

This value is just short of 140 140 , hence the answer of 139 139 .

Ash Crow
Aug 12, 2018

you should ve wrote that you must round it

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