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Difference between revisions of "2024 AMC 10B Problems/Problem 14"

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{{duplicate|[[2024 AMC 10B Problems/Problem 14|2024 AMC 10B #14]] and [[2024 AMC 12B Problems/Problem 9|2024 AMC 12B #9]]}}
 
{{duplicate|[[2024 AMC 10B Problems/Problem 14|2024 AMC 10B #14]] and [[2024 AMC 12B Problems/Problem 9|2024 AMC 12B #9]]}}
  
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==Problem==
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==Problem 9==
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A dartboard is the region B in the coordinate plane consisting of points <math>(x, y)</math> such that <math>|x| + |y| \le 8</math>. A target T is the region where <math>(x^2 + y^2 - 25)^2 \le 49</math>. A dart is thrown at a random point in B. The probability that the dart lands in T can be expressed as <math>\frac{m}{n} \pi</math>, where <math>m</math> and <math>n</math> are relatively prime positive integers. What is <math>m + n</math>?
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<math>
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\textbf{(A) }39 \qquad
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\textbf{(B) }71 \qquad
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\textbf{(C) }73 \qquad
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\textbf{(D) }75 \qquad
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\textbf{(E) }135 \qquad
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</math>
  
 
==Simple Coordinate Geometry==
 
==Simple Coordinate Geometry==

Revision as of 02:51, 14 November 2024

The following problem is from both the 2024 AMC 10B #14 and 2024 AMC 12B #9, so both problems redirect to this page.

Problem 9

A dartboard is the region B in the coordinate plane consisting of points $(x, y)$ such that $|x| + |y| \le 8$. A target T is the region where $(x^2 + y^2 - 25)^2 \le 49$. A dart is thrown at a random point in B. The probability that the dart lands in T can be expressed as $\frac{m}{n} \pi$, where $m$ and $n$ are relatively prime positive integers. What is $m + n$?

$\textbf{(A) }39 \qquad \textbf{(B) }71 \qquad \textbf{(C) }73 \qquad \textbf{(D) }75 \qquad \textbf{(E) }135 \qquad$

Simple Coordinate Geometry

\[|x|+|y| \le 8\] Inequalities of this form are well-known and correspond to a square in space with centre at origin and vertices at $(8, 0)$, $(-8, 0)$, $(0, 8)$, $(0, -8)$. The diagonal length of this square is clearly $16$, so it has an area of \[\frac{1}{2} \cdot 16 \cdot 16 = 128\] Now, \[(x^2 + y^2 - 25)^2 \le 49\] Converting to polar form, \[r^2 - 25 \le 7\] \[r \le \sqrt32\] And \[r^2 - 25 \ge -7\] \[r \ge \sqrt18\]

This corresponds to a ring in space with outer radius $\sqrt32$ and inner radius $\sqrt18$. Note that the outer circle is inscribed within the square, meaning it completely lies within the square.

Our probability, then, is \[\frac {\pi(32 - 18)}{128}\] \[= \frac{7\pi}{64}\] $m = 7$ and $n = 64$ \[m + n = 71\] So $\boxed{\textbf{(B) }71}$

See also

2024 AMC 10B (Problems • Answer Key • Resources)
Preceded by
Problem 13 		Followed by
Problem 15
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
2024 AMC 12B (Problems • Answer Key • Resources)
Preceded by
Problem 8 		Followed by
Problem 10
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

These problems are copyrighted © by the Mathematical Association of America, as part of the American Mathematics Competitions. AMC Logo.png

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