Question

A circle with integer radius $r$ is centered at $(r,r).$ Distinct line segments of length $c_i$ connect points $(0,a_i)$ to $(b_i,0)$ for $1\le i\le14$ and are tangent to the circle, where $a_i,$ $b_i,$ and $c_i$ are all positive integers and $c_1\le c_2\le\cdots\le c_{14}.$ What is the ratio $\dfrac{c_{14}}{c_1}$ for the least possible value of $r?$

$\dfrac{21}{5}$

$\dfrac{85}{13}$

$7$

$\dfrac{39}{5}$

$17$

Answer: E

Concepts:incircle, incenter, and inradius Pythagorean Triple factor counting

Difficulty rating: 2650

Solution:

The circle centered $(r,r)$ with radius $r$ is tangent to both axes. A segment from $(0,a)$ to $(b,0)$ with $a^2+b^2=c^2$ is tangent to it when $r$ equals either the inradius $\tfrac{a+b-c}{2}$ or the semiperimeter $\tfrac{a+b+c}{2}$ of the right triangle with legs $a,b.$

In the inradius case, put $x=a-2r$ and $y=b-2r.$ Then $xy=2r^2,$ and every positive divisor $x$ determines one oriented segment, with $a=x+2r,$ $b=2r^2/x+2r,$ and $c=x+2r^2/x+2r.$ Thus there are exactly $d(2r^2)$ such segments.

For $r=1,2,3,4,5,$ these counts are $2,4,6,6,6,$ and no semiperimeter case is possible because the smallest integer right triangle has semiperimeter $6.$ At $r=6,$ $d(72)=12,$ and the $3$ - $4$ - $5$ triangle contributes two more oriented segments. Hence $6$ is the least possible radius and gives exactly $14$ segments.

The two semiperimeter segments have $c=5.$ In the inradius family, $c=x+72/x+12$ is largest at $x=1$ or $72,$ giving $c=85.$ Therefore $c_1=5,$ $c_{14}=85,$ and $c_{14}/c_1=17.$

Thus, the correct answer is E.

2022 AMC 12A Problem 25

Answer: E

Difficulty rating: 2650

Solution:

Problem 25 in Other Years