Universal set $\xi$ is the set of all elements under consideration. $\in$ means 'is an element o

Question

Universal set $\xi$ is the set of all elements under consideration. $\in$ means 'is an element of'. $\notin$ means 'is not an element of'. Listing all elements in set notation, e.g. $A=\{1,3,4,5\}$. $n(A)$ means the number of the elements in the set $A$. Equal sets contain exactly the same elements. E.g. $A=\{1,3,4,5\}$, $B=\{5,3,4,1\}$, thus $A=B$ and $\mathrm{n}(A)=\mathrm{n}(B)$. The empty or null set is the set containing no element. E.g. $A=\{\space\}$ or $A=\varnothing$.

Accepted Answer

For $A=\{1,3,4,5\}$, $n(A)=4$.

Introduction to Set Notations

Introduction to Set Notations

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