Trigonometry

The branch of geometry in which the metric relationships between the elements of a triangle are described in terms of trigonometric functions (see, e.g., Cosine theorem; Sine theorem; Tangent formula), as well as formulas relating the trigonometric functions (so-called goniometry) are established. Trigonometry is considered both in Euclidean and non-Euclidean geometries. The trigonometry of a sphere in Euclidean space is called spherical trigonometry.

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Cf. also Plane trigonometry.