libm.info: acosh

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`acosh', `acoshf'--inverse hyperbolic cosine

*Synopsis*
     #include <math.h>
     double acosh(double X);
     float acoshf(float X);
   *Description*
`acosh' calculates the inverse hyperbolic cosine of X.  `acosh' is
defined as
      log(X + sqrt(X*X-1))
   X must be a number greater than or equal to 1.
   `acoshf' is identical, other than taking and returning floats.
*Returns*
`acosh' and `acoshf' return the calculated value.  If X less than 1,
the return value is NaN and `errno' is set to `EDOM'.
   You can change the error-handling behavior with the non-ANSI
`matherr' function.
*Portability*
Neither `acosh' nor `acoshf' are ANSI C.  They are not recommended for
portable programs.