Question

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 and lands at a random point in $B.$ The probability that the dart lands in $T$ can be expressed as $\dfrac{m}{n} \cdot \pi,$ where $m$ and $n$ are relatively prime positive integers. What is $m + n?$

$39$

$71$

$73$

$75$

$135$

Answer: B

Concepts:geometric probability annulus tangent line

Difficulty rating: 1660

Solution:

$B$ is the square $|x| + |y| \le 8,$ with area $2 \cdot 8^2 = 128.$ The target condition $(x^2 + y^2 - 25)^2 \le 49$ unpacks to $|x^2 + y^2 - 25| \le 7,$ that is $18 \le x^2 + y^2 \le 32,$ an annulus of area $\pi(32 - 18) = 14\pi.$ Does it fit inside $B?$ The distance from the origin to an edge $x + y = 8$ is $\tfrac{8}{\sqrt2} = 4\sqrt2 = \sqrt{32},$ exactly the outer radius, so yes, the annulus sits inside the square. The probability is $\tfrac{14\pi}{128} = \tfrac{7}{64}\pi,$ giving $m + n = 71.$ Therefore, the answer is B.

2024 AMC 10B Problem 14

Answer: B

Difficulty rating: 1660

Solution:

Problem 14 in Other Years