public static PV PV_Norm(PV v) { double l = v.Length(); return new PV(v.x / l, v.y / l, v.z / l); }
public static double PV_CosAngle2V(PV v1, PV v2) { return PV_DotProduct(v1, v2) / (v1.Length() * v2.Length()); }