Why is 1 excluded

This includes elements that are present in both A and B.

$$A = \{1, 2, 3, 4\}$$ $$B = \{3, 5, 6\}$$

The only element common to both sets is 3.

$$A \cap B = \{3\}$$

This includes all unique elements from $A \cap B$ or B.

$$(A \cap B) \cup B = \{3\} \cup \{3, 5, 6\}$$

Combining all unique elements from these two sets gives:

$$\{3, 5, 6\}$$

The number 1 is in set A, but it is not in $A \cap B$ (because it's not in B), and it is also not in set B itself. Therefore, 1 is not included in the union of $\{3\}$ and $\{3, 5, 6\}$.