Brianchon's Theorem
This theorem is known as a dual theorem of
Pascal 's theorem.
If lines ABCDEF lie on a conic, then lines
(AB)(DE), (BC)(EF), (CD)(FA) lie on one point.
If six lines lie on a conic, then the hexagon
which they form circumscribes the conic.
The proofs for these are exactly the duals
of the proofs for Pascal's theorem
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