Выпуклое множество

       public double AngelBeetwenVectors(double[] first, double[] second, double[] third) {
            int xCoordinate = 0;
            int yCoordinate = 1;
            double[] vector1 = new double[2] {  second[xCoordinate] - first[xCoordinate], second[yCoordinate] -  first[yCoordinate]};
            double vector1Length = Math.Sqrt(Math.Pow(vector1[xCoordinate], 2) + Math.Pow(vector1[yCoordinate], 2));
            double[] vector2 = new double[2] {  third[xCoordinate] - second[xCoordinate], third[yCoordinate] - second[yCoordinate] };
            double vector2Length = Math.Sqrt(Math.Pow(vector2[xCoordinate], 2) + Math.Pow(vector2[yCoordinate], 2));
            double scalar = vector1[xCoordinate] * vector2[xCoordinate] + vector1[yCoordinate] * vector2[yCoordinate];
            double cos = scalar / (vector1Length * vector2Length);
            double tg = Math.Sqrt(1.0 / (cos * cos) - 1);
            //double tg = (vector1[xCoordinate] * vector2[yCoordinate]- vector2[xCoordinate]* vector1[yCoordinate]) 
            //        / (vector1[xCoordinate] * vector2[xCoordinate] + vector2[yCoordinate] * vector1[yCoordinate]);
            double atan = Math.Atan(tg) / Math.PI * 180;
            //if (atan < 0)
            //    return atan + 180;
            //else
            //    return atan;
            return Math.Abs(atan);
        }