Trigonometric functions example) and another of the sides remains the same. If the hypotenuse is twice as long, so are the sides. It is these ratios that the trigonometric functions express. To define the trigonometric functions for the angle "A", start with any right triangle that contains the angle "A". The three sides of the triangle are named as follows: In ordinary Euclidean geometry, according to the triangle postulate, the inside angles of every triangle total 180° ( radians). Therefore, in a right-angled triangle, the two non-right angles total 90° ( radians), so each of these angles must be in the range
Sphere Many theorems from classical geometry hold true for spherical geometry as well, but not all do because the sphere fails to satisfy some of classical geometry's postulates, including the parallel postulate. In spherical trigonometry, angles are defined between great circles. Spherical trigonometry differs from ordinary trigonometry in many respects. For example, the sum of the interior angles of a spherical triangle always exceeds 180 degrees. Also, any two similar spherical triangles are congruent. In their book "Geometry and the Imagination" David Hilbert and Stephan Cohn-Vossen describe eleven properties of the sphere and discuss whether these properties uniquely determine the sphere.