
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
The number of diagonals in a convex polygon with $n$ sides is denoted as $a_n$ (where $n \geq 4$). For example, $a_4 = 2$. Then:  
① $a_5 =$    ;  
② $a_6 - a_5 =$    ;  
③ $a_{n+1} - a_n =$    . (For $n \geq 4$, express in an algebraic formula involving $n$)

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