Question

Let $ABCDE$ be an equiangular convex pentagon of perimeter $1.$ The pairwise intersections of the lines that extend the sides of the pentagon determine a five-pointed star polygon. Let $s$ be the perimeter of this star. What is the difference between the maximum and the minimum possible values of $s?$

$0$

$\dfrac{1}{2}$

$\dfrac{\sqrt5 - 1}{2}$

$\dfrac{\sqrt5 + 1}{2}$

$\sqrt5$

Answer: A

Concepts:equiangular polygon isosceles triangle invariant

Difficulty rating: 1890

Solution:

An equiangular pentagon has all interior angles $108^\circ,$ so each point of the star is an isosceles triangle with base angles $72^\circ$ and apex $36^\circ.$ By the equal base angles, each point contributes two sides that are the same fixed multiple $c$ of the pentagon side it rests on. Summing over the five points, the star perimeter equals $2c\cdot(\text{pentagon perimeter}) = 2c,$ independent of the individual side lengths. So $s$ is constant, and the difference between its maximum and minimum values is $0.$ Thus, the correct answer is A.

2013 AMC 12B Problem 16

Answer: A

Difficulty rating: 1890

Solution:

Problem 16 in Other Years