2. a>5 and A+4 >; a a+4 & lt； -1 and a+4 >; Answer.

3. Find the solution of the equation in set A, then bring it into B to find A, and then find the inverse solution, excluding the equation whose solution in set B is not the solution of the equation in set A. ..

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Set is a basic concept in mathematics and the research object of set theory. The basic theory of set theory was not founded until19th century. In the simplest way, defined by the most primitive set theory-naive set theory, a set is "a bunch of things". The "things" in a set are called elements. If x is an element of set a, it is recorded as x ∈ a.