**Trigonometry** is a branch of mathematics which deals with triangles, particularly triangles in a plane where one angle of the triangle is 90 degrees (right angled triangles). Triangles on a sphere are also studied, in spherical trigonometry. Trigonometry specifically deals with the relationships between the sides and the angles of triangles, that is, the trigonometric functions, and with calculations based on these functions.

Trigonometry has important applications in many branches of pure mathematics as well as of applied mathematics and, consequently, much of science. Trigonometry is usually taught in secondary schools, often in a precalculus course.

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