acosh

/*
 *  call-seq:
 *     Math.acosh(x)    => float
 *  
 *  Computes the inverse hyperbolic cosine of <i>x</i>.
 */

static VALUE
math_acosh(obj, x)
    VALUE obj, x;
{
    double d;

    Need_Float(x);
    errno = 0;
    d = acosh(RFLOAT(x)->value);
    domain_check(d, "acosh");
    return rb_float_new(d);
}