An archaic or variant spelling of cosine, the trigonometric function that measures the ratio of the adjacent side to the hypotenuse in a right triangle.
From co- (complementary) + sine (from Latin sinus, originally meaning bosom or curve). This older spelling appears in 18th-century mathematical texts.
Old math textbooks sometimes used 'cosin' instead of 'cosine,' showing how scientific terminology standardized over time as mathematics became more universal.
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