1. Given $A \cup B=\left\{a_{1}, a_{2}, a_{3}\right\}$, when $A \neq B$, $(A, B)$ and $(B, A)$ are considered different pairs, then the number of such $(A, B)$ pairs is
$\qquad$.


Answer 26.
Analysis Since sets $A, B$ are both subsets of $A \cup B$, $A \neq B$ and $A \cup B=\left\{a_{1}, a_{2}, a_{3}\right\}$. When $A=\varnothing$, $B$ has 1 way of being chosen;
When $A$ is a singleton, $B$ has 2 ways of being chosen;
When $A$ is a two-element set, $B$ has 4 ways of being chosen;
When $A$ is a three-element set, $B$ has 7 ways of being chosen;
Therefore, there are $1+3 \times 2+3 \times 4+7=26$ different $(A, B)$ pairs.