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This page uses JavaSketchpad, a World-Wide-Web component of The Geometer's Sketchpad. Copyright © 1990-2001 by KCP Technologies, Inc. Licensed only for non-commercial use.

By moving the vertices of the gray triangle, you can change where the internal circle is placed. (It is defined by the midpoints of the sides.) By moving point D around, you can see that any point on the outer circle can be the vertex of an inscribed triangle with midoints of edges on the same second circle. This porism was inspired by a question from Steve Gray. It seems to be true for higher order n-gons, but I have only proved it for n = 3 and 4.

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midpoint porism

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