public static double dot(Point2d P1, Point2d P2) // inner product 2
{
return(P1.X * P2.X + P1.Y * P2.Y);
}
/// <summary>
/// 拷贝构造函数
/// </summary>
/// <param name="P">二维点P</param>
public Point2d(Point2d P)
{
X = P.X;
Y = P.Y;
}