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**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