如果 
和 
是直线，且 
和 
相交于 
，且 
，则 
。
Pedoe (1976) 讨论了该定理的起源和一些历史，他将其归功于 L. M. Urquhart。 然而，de Morgan 在 1841 年发表了该定理的证明，并且该定理可以被视为追溯到 1860 年 Chasles 的一个结果的极限情况 (Deakin 1981, Deakin 1982, Hajja 2006)。
厄克特定理
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参考文献
Deakin, M. A. B. "The Provenance of Urquhart's Theorem." Austral. Math. Soc. Gaz. 8, 26, 1981.Deakin, M. A. B. Addendum to "The Provenance of Urquhart's Theorem." Austral. Math. Soc. Gaz. 9, 100, 1982.Deakin, M. A. B. "Yet More on Urquhart's Theorem." http://www.austms.org.au/Publ/Gazette/1997/Apr97/letters.html.Eustice, D. "Urquhart's Theorem and the Ellipse." Crux Math. (Eureka), 2, 132-133, 1976.Grossman, H. "Urquhart's Quadrilateral Theorem." Math. Teacher 66, 643-644, 1973.Hajja, M. "An Elementary Proof of the Most 'Elementary' Theorem of Euclidean Geometry." J. Geom. Graphics 8, 17-22, 2004.Hajja, M. "A Very Short and Simple Proof of 'the Most Elementary Theorem' of Euclidean Geometry." Forum Geom. 6, 167-169, 2006.Kazarinoff, N. D. "Geometric Inequalities." Washington, DC: Math. Assoc. Amer., 1961.Konhauser, J. D. E.; Velleman, D.; and Wagon, S. Which Way Did the Bicycle Go? And Other Intriguing Mathematical Mysteries. Washington, DC: Math. Assoc. Amer., 1996.Pedoe, D. "The Most 'Elementary' Theorem of Euclidean Geometry." Math. Mag. 49, 40-42, 1976.Sauvé, L. "On Circumscribable Quadrilaterals." Crux Math. (Eureka), 2, 63-67, 1976.Sokolowsky, D. "Extensions of Two Theorems by Grossman." Crux Math. (Eureka) 2, 163-170, 1976.Sokolowsky, D. "A 'No-Circle' Proof of Urquhart's Theorem." Crux Math. (Eureka) 2, 133-134, 1976.Trost, E. and Breusch, R. Problem 4964. Amer. Math. Monthly 68, 384, 1961.Trost, E. and Breusch, R. Solution to Problem 4964. Amer. Math. Monthly 69, 672-674, 1962.Williams, K. S. "Pedoe's Formulation of Urquhart's Theorem." Ontario Math. Gaz. 15, 42-44, 1976.Williams, K. S. "On Urquhart's Elementary Theorem of Euclidean Geometry." Crux Math. (Eureka) 2, 108-109, 1976.在 中被引用
厄克特定理请引用为
Eric W. Weisstein. "厄克特定理." 来自 Web 资源. https://mathworld.net.cn/UrquhartsTheorem.html