外正割
參考文獻

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↑  Swanson, Todd; Andersen, Janet; Keeley, Robert.  (PDF). . Harcourt Brace & Company（英语：）. 1999: 344  [2015-11-12]. （原始内容 (PDF)存档于2003-06-17）.
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↑  Galilei, Galileo. Finocchiaro, Maurice A. , 编. . University of California Press. 1997-05-25: 184 (n130), 184 (n135), 192 (n158) [1632]  [2017-07-30]. ISBN 9780520918221. […] Galileo's word is segante (meaning secant), but he clearly intends exsecant; an exsecant is defined as the part of a secant external to the circle […]  含有內容需登入查看的頁面 (link)
↑  Hawking, Stephen William (编). . Philadelphia, USA: Running Press（英语：）. 2002  [2017-07-31]. ISBN 0-7624-1698-X. LCCN 2002100441.  含有內容需登入查看的頁面 (link)
↑  Stávek, Jiří. . Applied Physics Research (Prague, CZ: Canadian Center of Science and Education). 2017-03-10, 9 (2): 57–64 [2017-02-26]. ISSN 1916-9639. doi:10.5539/apr.v9n2p57 . ISSN 1916-9647.
↑  Haslett, Charles. Hackley, Charles W. , 编. . New York, USA: James G. Gregory, successor of W. A. Townsend & Co. (Stringer & Townsend). September 1855  [2017-08-13]. […] Still there would be much labor of computation which may be saved by the use of tables of external secants and versed sines, which have been employed with great success recently by the Engineers on the Ohio and Mississippi Railroad（英语：）, and which, with the formulas and rules necessary for their application to the laying down of curves, drawn up by Mr. Haslett, one of the Engineers of that Road, are now for the first time given to the public. […] In presenting this work to the public, the Author claims for it the adaptation of a new principle in trigonometrical analysis of the formulas generally used in field calculations. Experience has shown, that versed sines and external secants as frequently enter into calculations on curves as sines and tangents; and by their use, as illustrated in the examples given in this work, it is believed that many of the rules in general use are much simplified, and many calculations concerning curves and running lines made less intricate, and results obtained with more accuracy and far less trouble, than by any methods laid down in works of this kind. The examples given have all been suggested by actual practice, and will explain themselves. […] As a book for practical use in field work, it is confidently believed that this is more direct in the application of rules and facility of calculation than any work now in use. In addition to the tables generally found in books of this kind, the author has prepared, with great labor, a Table of Natural and Logarithmic Versed Sines and External Secants, calculated to degrees, for every minute; also, a Table of Radii and their Logarithms, from 1° to 60°. […]  1856 edition
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