Trigonometry (from Ancient Greek ???????? (tríg?non) 'triangle' and ?????? (métron) 'measure') is a branch of mathematics concerned with relationships between angles side lengths triangles. In particular the trigonometric functions relate right triangle ratios its lengths. The field emerged in Hellenistic world during 3rd century BC from applications geometry to astronomical studies. Greeks focused on calculation chords while mathematicians India created earliest-known tables values for (also called functions) such as sine. Throughout history trigonometry has been applied areas geodesy surveying celestial mechanics navigation. known many identities. These identities are commonly used rewriting trigonometrical expressions aim simplify an expression find more useful form or solve equation