Question

Please i don't understand set theory in mathematics

Answer 1

Set theory is the mathematical theory of well-determined collections, called sets, of objects that are called members, or elements, of the set. ... The axioms of set theory imply the existence of a set-theoretic universe so rich that all mathematical objects can be construed as sets

For example, if A = {1, 2, 3} and B = {1, 2, 3, 4, 5} then A Í B (Í means is a subset of). If A = {1, 2, 4, 8}, then n(A) = 4. This is because n(A) means the number of members in set A. The universal set is the set of all sets.