char
float_arctan(
floatnum x,
int digits)
{
if (!chckmathparam(x, digits))
return 0;
_arctan(x, digits);
return 1;
}
/* evaluates arccos(1+x) for -0.5 <= x <= 0
arccos(1+x) = arctan(sqrt(-x*(2+x))/(1+x))
the relative error of a 100 digit result is < 5e-100 */
void
_arccosxplus1lt0_5(
floatnum x,
int digits)
{
floatstruct tmp;
float_create(&tmp);
float_add(&tmp, x, &c2, digits+1);
float_mul(x, x, &tmp, digits+1);
float_setsign(x, 1);
float_sqrt(x, digits);
float_sub(&tmp, &tmp, &c1, digits);
float_div(x, x, &tmp, digits+1);
_arctan(x, digits);
float_free(&tmp);
}
