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## Equality

Definition 3.2.6   Two sets and are equal if and .

This is exactly the meaning of a sentence such that: ``the set of real solutions of the quadratic equation is ''. If the number is a solution of the given equation, then or ; conversely, if or , then is a solution of the given quadratic equation. If two sets and are not equal, we denote . For example, . Note that it suffices to find one element who belongs to one set and not to the other to prove inequality of two sets.

root 2002-06-10