Trigonometry Triangle to Show Sine, Cosine, and Tangent
Right triangle OCA, inside of Circle O is used to show that side AC is "opposite" O and side OC is "adjacent" to O. OA is the hypotenuse. Sine is defined as the ratio of the opposite side to the hypotenuse (AC/OA). Cosine is defined as the ratio of the adjacent side to the hypotenuse (OC/OA), and Tangent is defined as the ratio of the opposite side to the adjacent side (DB/OB).
International Correspondence Schools I.C.S. Reference Library Mathematics (Scranton: International Textbook Company, 1905) Geometry & Trigonometry Pg. 25
Courtesy the private collection of Roy Winkelman