This page is dedicated to the proof and understanding of the recent Lester Circle Theorem.  First a definition:
 
Lester's Theorem: 
The Circumcenter, the nine-point center, and the first and second Fermat points of a triangle lie on a circle.
 

First, we need to know what the different parts of the construction are.  We will start with the Circumcenter, then go to the 9-point center, followed by the First and Second Fermat points.

 
 
Construction Note: 
In all of the Sketch's below, you can drag around points A, B, and C.  This will give you an infinite number of examples of the properties being shown. 

If you are lost or want to reset the sketch, just hit the letter "r".  For animations, hitting "<" will slow down the animation, and ">" will speed it up.

  
 
 
 
 
The circumcenter of a circle is obtained by finding the intersection of the three perpendicular bisectors of each side of the triangle.  Drag points A, B, and C to see how different triangles have different circumcenters. 
 
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The 9-point circle is constructed by drawing a circle that passes through each of the midpoints of the sides of a triangle. Drag points A, B, and C to see how different triangles have different 9-point centers.
 
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The First Fermat point is found like this.  Start with one side of the triangle.  Treat that side as the base of an equilateral triangle on the outside of the original triangle.  It will have a vertex that was not on the original triangle.  Connect that point with the other original vertex.  Repeat that for all three sides of the original triangle.  Where these three lines intersect is the 1st Fermat point. Drag points A, B, and C to see how different triangles have different 1st Fermat Points.
 
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The second Fermat point is found in the same manner as the first, but this time the triangles are drawn inward. Drag points A, B, and C to see how different triangles have different 2nd Fermat Points
 
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Finally, here are all four points together on one construction.  Drag point A, B, or C to see how different triangles still obey Lerster's Circle Theorem.
 
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 Please mail comments and suggestions to me at  tahlsch@ops.org
 

This is a prototype of Java Sketchpad, a Worldwide Web component of The Geometer's Sketchpad. Copyright ©1990-1997 by Key Curriculum Press, Inc. All rights reserved. Portions of this work are being funded by the National Science Foundation (awards DMI 9561674 & 9623018).