Maths Olympiad Prep

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Geometry Difficulty 4.8 AIME Prove it United States

Problem:

Given three points AA, BB, CC known to lie on a circle, prove that one can reconstruct the original circle with a straightedge and compass.

Solution

Solution:

Here is a suitable procedure: First draw circles centered at AA and BB with radius ABAB and join their two points of intersection to create the perpendicular bisector of ABAB. This line contains all points that have the same distance from AA and BB, so the center of a circle through AA and BB lies on it. Then, similarly construct the perpendicular bisector of ACAC. Because AA, BB, and CC are known to lie on a circle, the two perpendicular bisectors are not parallel (or else no point could have the same distance from AA, BB, and CC), so they must meet at a point OO. Draw a circle centered at OO with radius OAOA.

Since OO lies on both perpendicular bisectors, this circle passes through BB and CC. Finally, any other circle through the same three points would have to have its center on both perpendicular bisectors, and hence at OO. Its radius must equal OAOA, implying that it coincides with the constructed circle.

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