public interface ISquared//что-то, имеющее площадь
    {        
        double GetSquare();
    }

    public class A : ISquared//треугольник по координатам вершин
    {
        private double x1;
        private double x2;
        private double x3;

        private double y1;
        private double y2;
        private double y3;
                
        public A(double x1, double y1, double x2, double y2, double x3, double y3)
        {
            this.x1 = x1;
            this.y1 = y1;
            this.x2 = x2;
            this.y2 = y2;
            this.x3 = x3;
            this.y3 = y3;
        }
        public virtual double GetSquare() 
        {
            return 0.5 * ( (x1 - x3) * (y2 - y3) - (x2 - x3) * (y1 - y3) ); 
        }
    }

    public class B : ISquared //равносторонний треугольник
    {
        private double a;

        public B(double a)
        {
            this.a = a;
        }
        public double GetSquare() 
        {
            return a * a * Math.Sqrt(3) / 4; 
        }
    }

    public class C : ISquared //равнобедренный треугольник
    {
        private double a;
        private double alpha;

        public C(double a, double alpha)
        {
            this.a = a;
            this.alpha = alpha;
        }
        public double GetSquare() 
        {
            return a * a * Math.Sin(alpha); 
        }
    }

    public class D : ISquared //прямоугольный треугольник
    {
        private double a;
        private double b;
        public D(double a, double b)
        {
            this.a = a;
            this.b = b;
        }
        public double GetSquare() 
        { 
            return a * b / 2; 
        }
    }