3 4 5 makes a right-angled triangle 3 5 4 makes a right-angl

Just keep inputting sets of 3 integers for each triangle you want to test.

When you've finished, enter -1 for either the length, base or hypotenuse.

There's a list of Pythagorean triples (as they're called) here:

http://en.wikipedia.org/wiki/Pythagorean_triple

Try this:

using System;

class Program
{
   static void Main()
   {
       Console.WriteLine("Enter data for triangle or -1 to quit\n");
       while(true)
       {    
          Console.Write("Length      : ");
          int  l = int.Parse(Console.ReadLine());
          if (l == -1) return;
          Console.Write("Base        : ");
          int  b = int.Parse(Console.ReadLine());
          if (b == -1) return;
          Console.Write("Hypotenuse  : ");
          int  h = int.Parse(Console.ReadLine());
          if (h == -1) return;
          bool result = IsRightAngledTriangle(l, b, h);          
          if (result)
          {
             Console.WriteLine("It's a right angled triangle");
          }
          else
          {
             Console.WriteLine("It's not a right angled triangle");
          }
          Console.WriteLine();
       }    
   }

   static bool IsRightAngledTriangle(int l, int b, int h)
   {
      return h * h == l * l + b * b; 
   }
}