Difference between revisions of "2002 OIM Problems/Problem 2"

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== Problem ==
 
== Problem ==
Let <math>C</math> and <math>D</math> be two points on the semicircle of diameter <math>AB</math> such that <math>B</math> and <math>C</math> are in different semiplanes with respect to the line <math>AD</math>. Let <math>M</math>, <math>N</math> and <math>P</math> denote the midpoints of <math>AC</math>, <math>DB</math> and <math>CD</math>, respectively. Let <math>O_A</math> and <math>O_B</math> be the circumcenters of the triangles <math>ACP</math> and <math>BDP</math>. Show that the lines <math>O_AO_B</math> and <math>MN</math> are parallel.
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Given any set of 9 points in the plane of which there are not three collinear, show that for each point <math>P</math> of the set, the number of triangles that have as vertices to three of the remaining eight points and to <math>P</math> inside it, it is even.
  
 
~translated into English by Tomas Diaz. ~orders@tomasdiaz.com
 
~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Latest revision as of 03:39, 14 December 2023

Problem

Given any set of 9 points in the plane of which there are not three collinear, show that for each point $P$ of the set, the number of triangles that have as vertices to three of the remaining eight points and to $P$ inside it, it is even.

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Solution

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See also

https://www.oma.org.ar/enunciados/ibe18.htm