 Archeological research also provides evidence of trigonometry used in other unique hydrological structures dating back to 4 B.C.E. The first recorded use of trigonometry came from the Hellenistic mathematician Hipparchus c. 150 B.C.E., who compiled a trigonometric table using the sine for solving triangles. Students often use mnemonics to remember facts and relationships in trigonometry. The mathematician Bartholemaeus Pitiscus published an influential work on trigonometry in 1595 which may have coined the word "trigonometry" itself.

image: www.maa.org Trigonometry has applications in both pure mathematics and in applied mathematics, where it is essential in many branches of science and technology. The thirteenth century Persian mathematician Nasir al-Din Tusi, along with Bhaskara, was probably the first to treat trigonometry as a distinct mathematical discipline. A branch of trigonometry, called spherical trigonometry, studies triangles on spheres, and is important in astronomy and navigation. Trigonometry deals with relationships between the sides and the angles of triangles and with the trigonometric functions, which describe those relationships. Persian mathematician Omar Khayyбm (1048-1131) combined trigonometry and approximation theory to provide methods of solving algebraic equations by geometrical means. The ancient Sinhalese in Sri Lanka, when constructing reservoirs in the Anuradhapura kingdom, used trigonometry to calculate the gradient of the water flow. Nasir al-Din Tusi in his Treatise on the Quadrilateral was the first to list the six distinct cases of a right angled triangle in spherical trigonometry. Trigonometry was developed for use in sailing as a navigation method used with astronomy. The origins of trigonometry can be traced to the civilizations of ancient Egypt, Mesopotamia and the Indus Valley (India), more than 4000 years ago.