Wolfram Demonstrations Projecthttp://demonstrations.wolfram.com/An Open Resource of Interactive Mathematica VisualizationsCopyright 2009 Wolfram Demonstrations Project & ContributorsAn Example of Subtraction of Negative Numbershttp://demonstrations.wolfram.com/AnExampleOfSubtractionOfNegativeNumbers/]]>http://demonstrations.wolfram.com/AnExampleOfSubtractionOfNegativeNumbers/