GeometryDifficulty 5.1AIME, harderFind the answerUnited States
Problem:
Regular hexagon ABCDEF has side length 2. Circle ω lies inside the hexagon and is tangent to segments AB and AF. There exist two perpendicular lines tangent to ω that pass through C and E, respectively. Given that these two lines do not intersect on line AD, compute the radius of ω.
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:
Let O be the center of ω, and let the two tangent lines intersect at P. Note that O lies on the external angle bisector of ∠CPE because the tangents are symmetric about line PO. Additionally, O lies on the perpendicular bisector of CE by symmetry. By Fact 5, COPE is cyclic and ∠COE=90∘. To finish, observe that ∠COD=45∘. Dropping the altitude CH down to AD gives OH=CH=3. So, AO=AH−OH=3−3. The desired answer is then 23⋅AO=[233−3].
Solution 2: Another way to get ∠COE=90∘ is as follows. Let ω meet the tangents from C and E at Q and R, respectively. Observe OC=OE (as O lies on the perpendicular bisector of CE) and OQ=OR, so △OCQ≅△OER. Then ∠COE=∠QOR=90∘.
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