如果 
和 
是直線，且 
和 
相交於 
，且 
，則 
。
Pedoe (1976) 討論了該定理的起源和一些歷史，他將其歸功於 L. M. Urquhart。 然而，de Morgan 在 1841 年發表了該定理的證明，並且該定理可以被視為追溯到 1860 年 Chasles 的一個結果的極限情況 (Deakin 1981, Deakin 1982, Hajja 2006)。
厄克特定理
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參考文獻
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
厄克特定理請引用為
Eric W. Weisstein. "厄克特定理." 來自 Web 資源. https://mathworld.tw/UrquhartsTheorem.html