double HalfLength(AcGePoint3d pt1, AcGePoint3d pt2)
{
double radius = 0;
double len1 = ( pt2.y - pt1.y);
double len2 = ( pt2.x - pt1.x);
radius = sqrt((len1 * len1) + (len2 * len2))/2;
return radius;
}
原文：http://www.cnblogs.com/pengjun-shanghai/p/4786378.html