Inscriptible Quadrilateral: An Illustration

The applet below illustrates the theorem of Pitot.

For a quadrilateral ABCD circumscribed around a circle the sums of opposite sides are equal:

AB + CD = BC + DA.

This applet requires Sun's Java VM 2 which your browser may perceive as a popup. Which it is not. If you want to see the applet work, visit Sun's website at, download and install Java VM and enjoy the applet.

What if applet does not run?

The commonly given proof of the converse contains a subtle flaw which is not at all simple to fix. But an additional proof smoothes the wrinkles.

Inscriptible (tangential, circumscribed) Quadrilateral

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Copyright © 1996-2018 Alexander Bogomolny


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