ETYM For co. sinus, an abbrev. of Latin complementi sinus.
(Homonym: cosign) Ratio of the adjacent side to the hypotenuse. In trigonometry, a function of an angle in a right triangle found by dividing the length of the side adjacent to the angle by the length of the hypotenuse (the longest side). It is usually shortened to cos. The two non-right angles of a right triangle add up to 90ş and are, therefore, described as complementary angles (or co-angles). If the two non-right angles are a and b, it may be seen that sin a = cos b sin b = cos a. Therefore, the sine of each angle equals the cosine of its co-angle. For example, if the co-angles of a triangle are 30ş and 60ş sin 30ş = cos 60ş = 0.5 sin 60ş = cos 30ş = 0.8660.