lab3/ConsoleApp1/ConsoleApp9/Program.cs

int[] x = new int[4];
int[] y = new int[4];
for (int i = 0; i < 4; ++i)
{
    Console.WriteLine("Введите (x" + i.ToString() + ",y" + i.ToString() + "): ");
    x[i] = Console.Read();
    y[i] = Console.Read();
    Console.ReadLine();
}

int a = (x[1] - x[0]) * (y[2] - y[1]) - (x[2] - x[1]) * (y[1] - y[0]);
int b = (x[2] - x[0]) * (y[3] - y[2]) - (x[3] - x[2]) * (y[2] - y[0]);
int c = (x[3] - x[0]) * (y[1] - y[3]) - (x[1] - x[3]) * (y[3] - y[0]);

if ((a >= 0 && b >= 0 && c >= 0) || (a <= 0 && b <= 0 && c <= 0))
{
    Console.WriteLine("Принадлежит треугольнику");
}
else
{
    Console.WriteLine("Не принадлежит треугольнике");
}

Console.ReadKey();