A trigonometric function equal to 1 minus the sine of an angle, used in historical navigation and astronomy.
From 'cover' plus 'sine' (from Latin 'sinus' meaning curve or fold). The 'cover-' prefix indicates the complementary nature of this function to standard sine.
Before calculators and computers, mathematicians created dozens of obscure trigonometric functions like coversine, versine, and haversine to make hand calculations faster—once computers arrived, we only kept the essential six trig functions!
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