
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given that $A = \{1, 2\}$, $B = \{3, a\}$, and the universal set $U = \{1, 2, 3, 4, 5\}$; since $A \cap B \neq \emptyset$, find $A \cup B$ and $C_U B$.

Present the answer in LaTex format: \boxed{Your answer}
user