Читать книгу SAT Math For Dummies with Online Practice - Mark Zegarelli - Страница 37

The first set of numbers you learn as a child are the natural numbers, or counting numbers, which are the positive whole numbers starting at 1 and continuing without end:

When you add or multiply any pair of natural numbers, the result is another natural number. Another way to say this is that the set of natural numbers is closed under both addition and multiplication. However, the natural numbers are not closed under subtraction or division, because when you subtract or divide a pair of natural numbers, the result isn’t always a natural number. For example: