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.
Cf. also Plane trigonometry.
Trigonometry. A.B. Ivanov (originator), Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Trigonometry&oldid=14165