see trigonometrytrigonometry [Gr.,=measurement of triangles], a specialized area of geometry concerned with the properties of and relations among the parts of a triangle. Spherical trigonometry is concerned with the study of triangles on the surface of a sphere rather than in the plane; it is .....Click the link for more information..
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
(sin), a trigonometric function. The sine of an acute angle in a right triangle is the ratio of the length of the leg opposite the angle to the length of the hypotenuse. The Indian mathematicians used the word jiva, whose literal meaning is “bowstring,” for the sine. The Arabs rendered jiva as jiba and subsequently used the reading jaib, which is a common word in Arabic meaning “curve,” “bosom,” or “fold” and corresponding to the Latin word sinus.
The sine of an angle A in a right triangle with hypotenuse of length c given by the ratio a / c, where a is the length of the side opposite A ; more generally, the sine function assigns to any real number A the ordinate of the point on the unit circle obtained by moving from (1,0) counterclockwise A units along the circle, or clockwise | A | units if A is less than 0. Denoted sin A.
In a right triangle, the ratio of the side opposite an acute angle (less than 90 degrees) and the hypotenuse. The cosine is the ratio between the adjacent side and the hypotenuse. These angular functions are used to compute circular movements.