Someone

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var ucgen = new Ucgen(new Nokta(3, 3), new Nokta(8, 1), new Nokta(12, 0));
            List<Nokta> noktalar = new List<Nokta>
            {
                new Nokta(3,3),
                new Nokta(0,0),
                new Nokta(12,1),
                new Nokta(3,4)
            };

            foreach (var item in noktalar)
            {
                if (UcgeninIcindemi(ucgen, item))
                    Console.WriteLine("içinde");
                else
                    Console.WriteLine("İçinde değil");
            }

            Console.ReadLine();
        }

        static bool AyniTaraftami(Isin isin, Nokta n1, Nokta n2)
        {
            return
            ((isin.B.X - isin.A.X) * (n1.Y - isin.A.Y) - (isin.B.Y - isin.A.Y) * (n1.X - isin.A.X) >= 0) &&
                ((isin.B.X - isin.A.X) * (n2.Y - isin.A.Y) - (isin.B.Y - isin.A.Y) * (n2.X - isin.A.X) >= 0);

        }

        static bool UcgeninIcindemi(Ucgen ucgen, Nokta n)
        {
            return
                (AyniTaraftami(new Isin(ucgen.A, ucgen.B), n, ucgen.C)) &&
                (AyniTaraftami(new Isin(ucgen.B, ucgen.C), n, ucgen.A)) &&
                (AyniTaraftami(new Isin(ucgen.C, ucgen.A), n, ucgen.B));
        }
    }

    class Nokta
    {
        public double X { get; set; }
        public double Y { get; set; }

        public Nokta(double x, double y)
        {
            X = x;
            Y = y;
        }
    }

    class Isin
    {
        public Nokta A { get; set; }
        public Nokta B { get; set; }

        public Isin(Nokta a, Nokta b)
        {
            A = a;
            B = b;
        }
    }

    class Ucgen
    {
        public Nokta A { get; set; }
        public Nokta B { get; set; }
        public Nokta C { get; set; }

        public Ucgen(Nokta a, Nokta b, Nokta c)
        {
            A = a;
            B = b;
            C = c;
        }
    }
}