function value = r8_acos ( x )

%*****************************************************************************80
%
%% r8_acos() evaluates the arc-cosine of an R8 argument.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    25 September 2011
%
%  Author:
%
%    Original FORTRAN77 version by Wayne Fullerton.
%    MATLAB version by John Burkardt.
%
%  Reference:
%
%    Wayne Fullerton,
%    Portable Special Function Routines,
%    in Portability of Numerical Software,
%    edited by Wayne Cowell,
%    Lecture Notes in Computer Science, Volume 57,
%    Springer 1977,
%    ISBN: 978-3-540-08446-4,
%    LC: QA297.W65.
%
%  Input:
%
%    real X, the argument.
%
%  Output:
%
%    real VALUE, the arc-cosine of X.
%
  value = 0.5 * pi - r8_asin ( x );

  return
end
