

   TTrriiggoonnoommeettrriicc FFuunnccttiioonnss

        cos(x)
        sin(x)
        tan(x)
        acos(x)
        asin(x)
        atan(x)
        atan2(y, x)

   DDeessccrriippttiioonn::

        These functions give the obvious trigonometric func-
        tions.  They respectively compute the cosine, sine,
        tangent, arc-cosine, arc-sine, arc-tangent, and the
        two-argument arc-tangent.

   NNoottee::

        All angles are in radians.

   SSeeee AAllssoo::

        `Log', `Math'.

   EExxaammpplleess::

        cos(0) == 1
        sin(3*pi/2) == cos(pi)
        x <- rnorm(99)
        all( sin(-x) == - sin(x))
        all( cos(-x) == cos(x))

        x <- 1:99/100
        all(Mod(1 - (cos(x) + 1i*sin(x)) / exp(1i*x)) < 1.1 * .Machine$double.eps)
         2* abs(1 - x / acos(cos(x))) / .Machine$double.eps #-- depends ?
        all(abs(1 - x / asin(sin(x))) <= .Machine$double.eps) # TRUE
        all(abs(1 - x / atan(tan(x))) <= .Machine$double.eps) # TRUE

