To determine the number of elements in the union of two sets $A$ and $B$, we use the following formula:

$n(A \cup B) = n(A) + n(B) - n(A \cap B)$

Where:

• $n(A)$ is the number of elements in set $A$,
• $n(B)$ is the number of elements in set $B$,
• $n(A \cap B)$ is the number of elements in the intersection of $A$ and $B$, i.e., the elements common to both sets.

To complete your formula:

$n(A \cup B) = n(A) + n(B) - n(A \cap B)$

If you have any questions or need further details on this formula, feel free to ask!

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Next Questions:

1. How does the formula change if the two sets $A$ and $B$ are disjoint?
2. Can this formula be applied to more than two sets? If so, how?
3. What is the relationship between the number of elements in the intersection and the union of two sets?
4. How do you calculate the number of elements in the union if one set is a subset of the other?
5. What is the significance of subtracting the intersection term in this formula?

Tip: The subtraction of the intersection term $n(A \cap B)$ is necessary to avoid counting the common elements twice when finding the union of two sets.