Pappus theorem deals with an hexagon ABCDEF inscribed in the degenerate conic of P

The theorem asserts that the intersection points of these three pairs are on a line.

The dual considers an hexagon of P

Another instance of the duality connecting two different figures offers the theorem of Brianchon, its dual being the theorem of Pascal.

Desargues.html

Duality.html

Menelaus.html

MenelausApp.html

PappusLines.html

Pascal.html

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