| |
| | Introduction to Sets |
 | | Is A a subset of B? And is B a subset of A? Well, we can't check every element in these sets, because they have an infinite number of elements. |
 | | A is a proper subset of B if and only if every element in A is also in B, and there exists at least one element in B that is not in A. This little piece at the end is only there to make sure that A is not a proper subset of itself. |
 | | On the contrary, {1, 2, 3} is a proper subset of {1, 2, 3, 4} because the element 4 is not in the first set. |
| www.mathsisfun.com /sets/sets-introduction.html (1772 words) |
|