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[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
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(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.