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Dao's Conic Theorem


DaosConicTheorem

Dao's conic theorem states that three intersections associated with a triangle inscribed in a conic section are collinear.

Let DeltaABC be inscribed in a nondegenerate conic section K, and let P be a point on neither K nor any of the three side lines of the triangle. Let AP, BP, and CP meet K again at A_1, B_1, and C_1, respectively. For a point D on K or on the polar of P with respect to K, define

A_2=DA_1 intersection BC
(1)
B_2=DB_1 intersection CA
(2)
C_2=DC_1 intersection AB.
(3)

Assume that the second intersections differ from the initial vertices and that all three displayed intersections are well-defined, finite, and distinct. Then A_2, B_2, and C_2 are collinear. Their line also contains P iff D lies on K (Tran 2018, p. 145).

Tran (2018, pp. 146-148) gives a synthetic proof and derives several classical theorems as special cases.


See also

Collinear, Conic Section, Polar

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References

Dao, T. O. "Generalizations of Some Famous Classical Euclidean Geometry Theorems." Int. J. Comput. Discov. Math. 1, 13-20, 2016. https://journal.bulgarian-kangaroo.eu/2016-3/Dao-Thanh-Oai-Generalizations-pp.12-20.pdf.Nguyen, M. H. "Synthetic Proof of Dao's Theorem." Global J. Adv. Res. Classical Modern Geom. 9, 20-23, 2020. https://geometry-math-journal.ro/pdf/Volume9-Issue1/4.pdf.Nguyen, N. G. "A Proof of Dao's Theorem." Global J. Adv. Res. Classical Modern Geom. 4, 102-105, 2015. https://geometry-math-journal.ro/pdf/Volume4-Issue2/5.pdf.Smith, G. "A Projective Simson Line." Math. Gaz. 99, 339-341, 2015. https://doi.org/10.1017/mag.2015.47.Tran, M. N. "A Purely Synthetic Proof of Dao's Theorem on a Conic and Its Applications." Int. J. Comput. Discov. Math. 3, 145-152, 2018. https://journal-1.eu/2018/Tran-Minh-Ngoc-Synthetic-Proof.pdf.

Cite this as:

Weisstein, Eric W. "Dao's Conic Theorem." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/DaosConicTheorem.html

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