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Trigonometry (from Greek trigōnon "triangle" + metron "measure"[1]) 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.