Concept
The exsecant (exsec, exs) and excosecant (excosec, excsc, exc) are trigonometric functions defined in terms of the secant and cosecant functions. They used to be important in fields such as surveying, railway engineering, civil engineering, astronomy, and spherical trigonometry and could help improve accuracy, but are rarely used today except to simplify some calculations. The exsecant, (Latin: secans exterior) also known as exterior, external, outward or outer secant and abbreviated as exsec or exs, is a trigonometric function defined in terms of the secant function sec(θ): The name exsecant can be understood from a graphical construction of the various trigonometric functions from a unit circle, such as was used historically. sec(θ) is the secant line , and the exsecant is the portion of this secant that lies exterior to the circle (ex is Latin for out of). A related function is the excosecant or coexsecant, also known as exterior, external, outward or outer cosecant and abbreviated as excosec, coexsec, excsc or exc, the exsecant of the complementary angle: Important in fields such as surveying, railway engineering (for example to lay out railroad curves and superelevation), civil engineering, astronomy, and spherical trigonometry up into the 1980s, the exsecant function is now little-used. Mainly, this is because the broad availability of calculators and computers has removed the need for trigonometric tables of specialized functions such as this one. The reason to define a special function for the exsecant is similar to the rationale for the versine: for small angles θ, the sec(θ) function approaches one, and so using the above formula for the exsecant will involve the subtraction of two nearly equal quantities, resulting in catastrophic cancellation. Thus, a table of the secant function would need a very high accuracy to be used for the exsecant, making a specialized exsecant table useful. Even with a computer, floating point errors can be problematic for exsecants of small angles, if using the cosine-based definition.
