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Difference between revisions of "2011 AMC 12A Problems/Problem 15"

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\textbf{(E)}\ \frac{13}{2} </math>
 
\textbf{(E)}\ \frac{13}{2} </math>
  
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== Solution ==
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== Solution 1 ==
  
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Let <math> ABCDE </math> be the pyramid with <math> ABCD </math> as the square base. Let <math> O </math> and <math> M </math> be the center of square <math> ABCD </math> and the midpoint of length <math> AB </math> respectively. Lastly, let the hemisphere be tangent to the triangular face <math> ABE </math> at <math> P </math>.
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Let <math> ABCDE </math> be the pyramid with <math> ABCD </math> as the square base. Let <math> O </math> and <math> M </math> be the center of square <math> ABCD </math> and the midpoint of side <math> AB </math> respectively. Lastly, let the hemisphere be tangent to the triangular face <math> ABE </math> at <math> P </math>.
  
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Notice that <math> \triangle EOM </math> has a right angle at <math> E </math>. Since the hemisphere be tangent to the triangular face <math> ABE </math> at <math> P </math>, <math>\angle EPO </math> is also <math> 90^{\circ} </math>. Hence <math> \triangle EOM </math> is similar to <math>\triangle EPO </math>.
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Notice that <math> \triangle EOM </math> has a right angle at <math> O </math>. Since the hemisphere is tangent to the triangular face <math> ABE </math> at <math> P </math>, <math>\angle EPO </math> is also <math> 90^{\circ} </math>. Hence <math> \triangle EOM </math> is similar to <math>\triangle EPO </math>.
  
 
<math> \frac{OM}{2} = \frac{6}{EP} </math>
 
<math> \frac{OM}{2} = \frac{6}{EP} </math>
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<math> OM = \frac{6}{\sqrt{6^2 - 2^2}} \times 2 = \frac{3\sqrt{2}}{2} </math>
 
<math> OM = \frac{6}{\sqrt{6^2 - 2^2}} \times 2 = \frac{3\sqrt{2}}{2} </math>
  
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The length of the square base is thus <math>2 \times \frac{3\sqrt{2}}{2} = 3\sqrt{2} \rightarrow \boxed{\textbf{}}</math>
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The length of the square base is thus <math>2 \times \frac{3\sqrt{2}}{2} = 3\sqrt{2} \rightarrow \boxed{\textbf{A}}</math>
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 +
== Solution 2 (Answer Choices) ==
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Consider a cross section of the pyramid such that it is a hemisphere inscribed inside of a triangle. Let <math>A</math> be the vertex furthest away from the hemisphere. Let <math>P</math> be a point where the hemisphere is tangent. Let <math>O</math> be the centre of the hemisphere. Triangle <math>AOP</math> is a right triangle, since <math>P</math> is ninety degrees. Apply the pythagorean formula to find side <math>AP</math> as <math>4\sqrt{2}.</math> Since we know the answer must have <math>\sqrt{2}</math> somewhere, we can remove choices B, D and E from selection. Choice C is simply the length of <math>AP</math>, which cannot be the desired length. Thus, the answer must be <math>\boxed{\textbf{A}}</math>.
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~ jaspersun
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==Video Solution==
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https://www.youtube.com/watch?v=u23iWcqbJlE
 +
~Shreyas S
  
 
== See also ==
 
== See also ==
 
{{AMC12 box|year=2011|num-b=14|num-a=16|ab=A}}
 
{{AMC12 box|year=2011|num-b=14|num-a=16|ab=A}}
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[[Category:Introductory Geometry Problems]]
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[[Category:3D Geometry Problems]]
 +
{{MAA Notice}}

Latest revision as of 22:14, 5 January 2024

Problem

The circular base of a hemisphere of radius $2$ rests on the base of a square pyramid of height $6$. The hemisphere is tangent to the other four faces of the pyramid. What is the edge-length of the base of the pyramid?

$\textbf{(A)}\ 3\sqrt{2} \qquad \textbf{(B)}\ \frac{13}{3} \qquad \textbf{(C)}\ 4\sqrt{2} \qquad \textbf{(D)}\ 6 \qquad \textbf{(E)}\ \frac{13}{2}$

Solution 1

Let $ABCDE$ be the pyramid with $ABCD$ as the square base. Let $O$ and $M$ be the center of square $ABCD$ and the midpoint of side $AB$ respectively. Lastly, let the hemisphere be tangent to the triangular face $ABE$ at $P$.

Notice that $\triangle EOM$ has a right angle at $O$. Since the hemisphere is tangent to the triangular face $ABE$ at $P$, $\angle EPO$ is also $90^{\circ}$. Hence $\triangle EOM$ is similar to $\triangle EPO$.

$\frac{OM}{2} = \frac{6}{EP}$

$OM = \frac{6}{EP} \times 2$

$OM = \frac{6}{\sqrt{6^2 - 2^2}} \times 2 = \frac{3\sqrt{2}}{2}$

The length of the square base is thus $2 \times \frac{3\sqrt{2}}{2} = 3\sqrt{2} \rightarrow \boxed{\textbf{A}}$

Solution 2 (Answer Choices)

Consider a cross section of the pyramid such that it is a hemisphere inscribed inside of a triangle. Let $A$ be the vertex furthest away from the hemisphere. Let $P$ be a point where the hemisphere is tangent. Let $O$ be the centre of the hemisphere. Triangle $AOP$ is a right triangle, since $P$ is ninety degrees. Apply the pythagorean formula to find side $AP$ as $4\sqrt{2}.$ Since we know the answer must have $\sqrt{2}$ somewhere, we can remove choices B, D and E from selection. Choice C is simply the length of $AP$, which cannot be the desired length. Thus, the answer must be $\boxed{\textbf{A}}$.

~ jaspersun

Video Solution

https://www.youtube.com/watch?v=u23iWcqbJlE ~Shreyas S

See also

2011 AMC 12A (Problems • Answer Key • Resources)
Preceded by
Problem 14 		Followed by
Problem 16
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All AMC 12 Problems and Solutions

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