// houlinbo QQ420021327
double A = Math.PI;
double dBulge = Math.Sin(A / 4) / Math.Cos(A / 4);
Polyline plCir = new Polyline();
plCir.AddVertexAt(0, new Point2d(pt1.X - R / 2, pt1.Y), dBulge, R, R);
plCir.AddVertexAt(1, new Point2d(pt1.X + R / 2, pt1.Y), dBulge, R, R);
plCir.AddVertexAt(2, new Point2d(pt1.X - R / 2, pt1.Y), dBulge, R, R);
// houlinbo QQ420021327
向内向外 用 + -