![\"\"](\"/userfiles/image/steps_images/step1.JPG\")
Select two different natural numbers and then determine whether
\the addition is a natural number.
\Try some examples.
\![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=cbf80e19b3083c81d60a9e72d52dd9aa.png\")
![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=91619371bf2cb390eb6bdb05727bdcc4.png\")
![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=203bd3dec7991269db17c2411781b1de.png\")
Since the sum of each pair of natural numbers is natural numbers, the set of natural
\numbers is closed under addition. ![\"\"](\"/userfiles/image/steps_images/solution.JPG\")
The set of natural numbers is closed under addition.