Versedsine and Coversedsine in Circle
The coversedsine is equal to 1 minus the sine. In this figure, AE is parallel to OB; hence, EO = AC = sine of angle AOC, when radius OA - 1. Therefore, Coversedsine = 1 - sine = 1 - AC/OA = 1 - AC = 1 - EO = ER, when OA = 1.
International Correspondence Schools I.C.S. Reference Library Mathematics (Scranton: International Textbook Company, 1905) Geometry & Trigonometry Pg. 27