using System;
 
public class Test
{
	public static void Main()
	{
		for (int n = 0; n <= 100; n += 5)
		{
			Console.WriteLine("n=" + n + " -> exp1: " + sigma(n) + " exp2: " + combination(n+1, 3));
		}
	}
 
	public static double sigma(int n)
	{
		double sum = 0.0;
		for (int i = 2; i <= n; i++)
		{
			sum += combination(i, 2);
		}
		return sum;
	}
 
	public static double combination(int n, int k)
	{
		return factorial(n) / (factorial(k) * factorial(n-k));
	}
 
	public static double factorial(int x)
	{
		if (x <= 1) { return 1; }
		return (double)x * factorial(x-1);
	}
}

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