Trigonometry (from Greek
trigōnon "triangle" + metron "measure") is a branch of mathematics that deals with triangles, particularly those
plane triangles in which one angle has 90 degrees (right triangles). Trigonometry deals with relationships between
the sides and the angles of triangles and with the trigonometric functions, which describe those relationships.
Trigonometry has applications in both pure mathematics and in applied mathematics, where it is essential in many
branches of science and technology. It is usually taught in secondary schools either as a separate course or as
part of a precalculus course. Trigonometry is informally called "trig".