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Berkeley Math Circle

Geometry Difficulty 4.5 AIME Prove it United States

Problem:

Given a triangle ABCABC, we draw three circles with respective diameters ABAB, BCBC, and CACA. Prove that there exists a point that is inside all three circles.

Solution

Solution:

We claim that the center II of the triangle's inscribed circle is such a point. To see this, note that
∠BIC=180−∠ICB+∠CBI=180−∠C+∠B2=180−180−A2=90+A2>90. \angle BIC = 180 - \angle ICB + \angle CBI = 180 - \frac{\angle C + \angle B}{2} = 180 - \frac{180 - A}{2} = 90 + \frac{A}{2} > 90.
Thus △BIC\triangle BIC is obtuse and if we drop a perpendicular BDBD from BB to CICI, then DD lies on the extension of ray CICI. The circle with diameter BCBC passes through DD since ∠BDC=90\angle BDC = 90, and thus II, an interior point of chord CDCD, lies inside the circle. By symmetry, we can conclude that II lies inside the other two circles as well.

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