One possible solution for problem 6 on the Project Euler website

ProblemSixEdit.c

/*
 * Project Name:            Problem Six Edit
 * Solution Name:           Problem Six Edit
 * Original creation date:  11/07/2011
 * Edit date:               18/01/2013
 * Programmer name:         Jamie Taylor (aka "GaProgMan")
 * File name:               ProblemSixEdit.c
 * 
 * Purpose of the project:
 * This code is my solution for "Problem Six" listed
 * on Project Euler.
 *    The original problem is listed in the comment
 * block below.
 *    Based on a comment made on my blog post conta-
 * ining this code, I've decided to optimise it
 * using the two forumlae below.
 *    Here is a link to the original post on my blog,
 * with the comment trail at the bottom of the page:
 *    http://wp.me/p19MGD-jh
 *    I've also decided to re-write this code in c
 * for the simple reason that it can be compiled on
 * a terminal with MingW or gcc - the PC I'm using
 * to writethis has no IDEs or compilers installed
 * on it.
 * Formulae used:
 * Square of the sum:
 *    Sn = [n(n + 1)] / 2
 * Sum of the squares:
 *    S^2n = [n(n + 1)(2n + 1)]/6
 *     Information on both of these formulae can be
 * found in many textbooks on mathematics and on many
 * mathematics websites.
 *    Here's an excellent website that provides a
 * useful look at the forumla for the Square of the
 * sum, and how to derive it:
 *    http://bit.ly/mD84iY
 *    Here is an excellent website that
 * provides a useful look at the forumla for the Sum
 * of the squares, and how to derive it:
 *    http://bit.ly/80s4Vg
 *
 *    GNU Copyright information
 *    Copyright 2011 Jamie Taylor <jamie@taylorj.org.uk>
 *
 *    This program is free software; you can redistribute
 *	it and/or modify it under the terms of the GNU General
 *	Public License as published by the Free Software
 *	Foundation; either version 2 of the License, or (at
 *	your option) any later version.
 *
 *	This program is distributed in the hope that it will
 *	be useful, but WITHOUT ANY WARRANTY; without even the
 *	implied warranty of MERCHANTABILITY or FITNESS FOR A
 *	PARTICULAR PURPOSE.  See the GNU General Public
 *	License for more details.
 *
 *	You should have received a copy of the GNU General
 *	Public License along with this program; if not, write
 *	to the Free Software Foundation, Inc., 51 Franklin
 *	Street, Fifth Floor, Boston, MA 02110-1301, USA.
*/
#include <stdio.h>

int main () {
  long sumOfSquares = 0;
  long squareOfSums = 0;
  long difference = 0;
  const int N = 100;

  sumOfSquares = (N * (N + 1)) / 2;
  squareOfSums = (N * (N + 1) * (2 * N + 1)) / 6;

  //by doing away with the calls to pow() in math.h, and just using x*x to
  //calculate a square the code becomes even more lean.
  difference = (sumOfSquares * sumOfSquares) - squareOfSums;

  printf("The difference between the square of the sums and the\nsum of the squares for all real numbers between 1 and %d is: %ld", N, difference);
  return 0;
}