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Definition: Trigonometry, informally called trig, is a branch of mathematics that deals with triangles, particularly triangles in a plane where one angle of the triangle is 90 degrees (right angled triangles). It specifically deals with the relationships between the sides and the angles of triangles; the trigonometric functions, and calculations based upon them. The insights of trigonometry permeate other branches of geometry, such as the study of spheres using spherical trigonometry. Trigonometry has important applications in many branches of pure mathematics as well as of applied mathematics and, consequently remains applicable in natural sciences. Trigonometry is usually taught in secondary schools, often in a precalculus course.