Chapter 2 - Set Theory - 2.3 Venn Diagrams and Set Operations - Exercise Set 2.3 - Page 83: 151

To find the number of the union of two finite sets $A$ and $B$ do this: 1- add the (number_of_element) in $A$ and (the number_of_element) in $B$ 2- then subtract the number of elements common to both sets ($A,\ B$) $\therefore n(A \cup B)=n(A)+n(B)-n(A \cap B)$ $n(A \cup B) \quad\quad\ \ \rightarrow$ number of elements in $A$ or $B$ $n(A)+n(B) \quad\rightarrow$ number of elements in $A$ plus number of elements in $B$ $n(A \cap B) \quad\quad\ \ \rightarrow$ number of common elements in both $A$ and $B$

To find the number of the union of two finite sets $A$ and $B$ do this: 1- add the (number_of_element) in $A$ and (the number_of_element) in $B$ 2- then subtract the number of elements common to both sets ($A,\ B$) $\therefore n(A \cup B)=n(A)+n(B)-n(A \cap B)$ $n(A \cup B) \quad\quad\ \ \rightarrow$ number of elements in $A$ or $B$ $n(A)+n(B) \quad\rightarrow$ number of elements in $A$ plus number of elements in $B$ $n(A \cap B) \quad\quad\ \ \rightarrow$ number of common elements in both $A$ and $B$