
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
Define an "equal sum sequence": In a sequence, if the sum of each term and its subsequent term is the same constant, then the sequence is called an equal sum sequence, and this constant is called the common sum. It is known that the sequence $\{a_n\}$ is an equal sum sequence with $a_1 = 2$ and the common sum is 5. Find the value of $a_{18}$ and the formula for the sum of the first $n$ terms of this sequence, denoted as $S_n$.

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