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.

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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