Maths Olympiad Prep

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, 2014

Geometry Difficulty 4.8 AIME Prove it United States

Problem:

Let ABCDEFABCDEF be a regular hexagon. Let PP be the circle inscribed in BDF\triangle BDF. Find the ratio of the area of circle PP to the area of rectangle ABDEABDE.

Solution

Solution:

π312\boxed{\frac{\pi \sqrt{3}}{12}} Let the side length of the hexagon be ss. The length of BDBD is s3s \sqrt{3}, so the area of rectangle ABDEABDE is s23s^{2} \sqrt{3}. Equilateral triangle BDFBDF has side length s3s \sqrt{3}. The inradius of an equilateral triangle is 3/6\sqrt{3} / 6 times the length of its side, and so has length s2\frac{s}{2}. Thus, the area of circle PP is πs24\frac{\pi s^{2}}{4}, so the ratio is πs2/4s23=π312\frac{\pi s^{2} / 4}{s^{2} \sqrt{3}}=\frac{\pi \sqrt{3}}{12}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.