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Difference between revisions of "2021 AMC 12A Problems/Problem 11"

(→‎Solution 2 (Detailed Explanation of Solution 1))
(→‎Solution 2 (Detailed Explanation of Solution 1))
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Reflecting <math>\overline{BC}</math> about the <math>y</math>-axis gives <math>\overline{BC'}.</math> Then, reflecting <math>\overline{CD}</math> over the <math>y</math>-axis gives <math>\overline{C'D'}.</math> Finally, reflecting <math>\overline{C'D'}</math> about the <math>x</math>-axis gives <math>\overline{C'D''},</math> as shown below.
 
Reflecting <math>\overline{BC}</math> about the <math>y</math>-axis gives <math>\overline{BC'}.</math> Then, reflecting <math>\overline{CD}</math> over the <math>y</math>-axis gives <math>\overline{C'D'}.</math> Finally, reflecting <math>\overline{C'D'}</math> about the <math>x</math>-axis gives <math>\overline{C'D''},</math> as shown below.
 +
 +
[[File:2021 AMC 12A Problem 11(2).png|center]]
  
 
It follows that <math>D''=(-7,-5).</math> The total distance that the beam will travel is  
 
It follows that <math>D''=(-7,-5).</math> The total distance that the beam will travel is  
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&=\boxed{\textbf{(C) }10\sqrt2}
 
&=\boxed{\textbf{(C) }10\sqrt2}
 
\end{align*}</cmath>
 
\end{align*}</cmath>
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−
[[File:2021 AMC 12A Problem 11(2).png|center]]
 
−
 
 
Graph in Desmos: https://www.desmos.com/calculator/lqatf2r9bu
 
Graph in Desmos: https://www.desmos.com/calculator/lqatf2r9bu
  

Revision as of 17:24, 14 February 2021

Problem

A laser is placed at the point $(3,5)$. The laser bean travels in a straight line. Larry wants the beam to hit and bounce off the $y$-axis, then hit and bounce off the $x$-axis, then hit the point $(7,5)$. What is the total distance the beam will travel along this path?

$\textbf{(A) }2\sqrt{10} \qquad \textbf{(B) }5\sqrt2 \qquad \textbf{(C) }10\sqrt2 \qquad \textbf{(D) }15\sqrt2 \qquad \textbf{(E) }10\sqrt5$

Diagram

File:2021 AMC 12A Problem 11.png

~MRENTHUSIASM (by Desmos: https://www.desmos.com/calculator/y6wyqok8gm)

Solution 1

Every time the laser bounces off a wall, instead we can imagine it going straight by reflecting it about the wall. Thus, the laser starts at $(3, 5)$ and ends at $(-7, -5)$, so the path's length is $\sqrt{10^2+10^2}=\boxed{\textbf{(C)} 10\sqrt{2}}$ ~JHawk0224

Solution 2 (Detailed Explanation of Solution 1)

Let $A=(3,5), D=(7,5), B$ be the point where the beam bounces off the $y$-axis, and $C$ be the point where the beam bounces off the $x$-axis.

Reflecting $\overline{BC}$ about the $y$-axis gives $\overline{BC'}.$ Then, reflecting $\overline{CD}$ over the $y$-axis gives $\overline{C'D'}.$ Finally, reflecting $\overline{C'D'}$ about the $x$-axis gives $\overline{C'D''},$ as shown below.

File:2021 AMC 12A Problem 11(2).png

It follows that $D''=(-7,-5).$ The total distance that the beam will travel is \begin{align*} AB+BC+CD&=AB+BC'+C'D' \\ &=AB+BC'+C'D'' \\ &=AD'' \\ &=\sqrt{((3-(-7))^2+(5-(-5))^2} \\ &=\sqrt{200} \\ &=\boxed{\textbf{(C) }10\sqrt2} \end{align*} Graph in Desmos: https://www.desmos.com/calculator/lqatf2r9bu

~MRENTHUSIASM

Video Solution by OmegaLearn (Using Reflections and Distance Formula)

https://youtu.be/e7tNtd-fgeo

~ pi_is_3.14

Video Solution by Hawk Math

https://www.youtube.com/watch?v=AjQARBvdZ20

See also

2021 AMC 12A (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 12 Problems and Solutions

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