Difference between revisions of "2017 AMC 10A Problems/Problem 11"

Revision as of 11:39, 12 February 2019

Problem

The region consisting of all points in three-dimensional space within 3 units of line segment $\overline{AB}$ has volume 216 $\pi$ . What is the length $\textit{AB}$ ?

$\textbf{(A)}\ 6\qquad\textbf{(B)}\ 12\qquad\textbf{(C)}\ 18\qquad\textbf{(D)}\ 20\qquad\textbf{(E)}\ 24$

Solution

In order to solve this problem, we must first visualize what the region contained looks like. We know that, in a three dimensional plane, the region consisting of all points within $3$ units of a point would be a sphere with radius $3$ . However, we need to find the region containing all points within 3 units of a segment. It can be seen that our region is a cylinder with two hemispheres on either end. We know the volume of our region, so we set up the following equation (the volume of our cylinder + the volume of our two hemispheres will equal $216 \pi$ ):

$\frac{4 \pi }{3} \cdot 3^3+9 \pi x=216 \pi$ , where $x$ is equal to the length of our line segment.

Solving, we find that $x = \boxed{\textbf{(D)}\ 20}$ .

Solution 2

Because this is just a cylinder and $2$ "half spheres", and the radius is $3$ , the volume of the $2$ half spheres is $4(3^3)/3 \pi = 36 \pi$ . Since we also know that the volume of this whole thing is $216 \pi$ , we do $216-36$ to get $180 \pi$ as the area of the cylinder. Thus the height is $180 \pi$ over the base, or $180 \pi/9\pi=20$ , $D$

Diagram for Solution

http://i.imgur.com/cwNt293.png

2017 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 10	Followed by Problem 12
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 10 Problems and Solutions

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.

@@ Line 12: / Line 12: @@
 Solving, we find that <math>x = \boxed{\textbf{(D)}\ 20}</math>.
+==Solution 2==
+Because this is just a cylinder and <math>2</math> "half spheres", and the radius is <math>3</math>, the volume of the <math>2</math> half spheres is <math>4(3^3)/3 \pi = 36 \pi</math>. Since we also know that the volume of this whole thing is <math>216 \pi</math>, we do <math>216-36</math> to get <math>180 \pi</math> as the area of the cylinder. Thus the height is <math>180 \pi</math> over the base, or <math>180 \pi/9\pi=20</math>,  <math>D</math>
 ==Diagram for Solution==

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