Maths Olympiad Prep

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Geometry Difficulty 5.1 AIME, harder Find the answer United States

Problem:

Regular hexagon ABCDEFABCDEF has side length 22. Circle ω\omega lies inside the hexagon and is tangent to segments AB\overline{AB} and AF\overline{AF}. There exist two perpendicular lines tangent to ω\omega that pass through CC and EE, respectively. Given that these two lines do not intersect on line ADAD, compute the radius of ω\omega.

Proposed by: Karthik Venkata Vedula

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

Figure 1
Let OO be the center of ω\omega, and let the two tangent lines intersect at PP. Note that OO lies on the external angle bisector of CPE\angle CPE because the tangents are symmetric about line POPO. Additionally, OO lies on the perpendicular bisector of CECE by symmetry. By Fact 5, COPECOPE is cyclic and COE=90\angle COE = 90^{\circ}. To finish, observe that COD=45\angle COD = 45^{\circ}. Dropping the altitude CHCH down to ADAD gives OH=CH=3OH = CH = \sqrt{3}. So, AO=AHOH=33AO = AH - OH = 3 - \sqrt{3}. The desired answer is then 32AO=[3332]\frac{\sqrt{3}}{2} \cdot AO = \left[\frac{3\sqrt{3} - 3}{2}\right].

Solution 2: Another way to get COE=90\angle COE = 90^{\circ} is as follows.
Let ω\omega meet the tangents from CC and EE at QQ and RR, respectively. Observe OC=OEOC = OE (as OO lies on the perpendicular bisector of CECE) and OQ=OROQ = OR, so OCQOER\triangle OCQ \cong \triangle OER. Then COE=QOR=90\angle COE = \angle QOR = 90^{\circ}.

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