
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 any two adjacent terms is always equal to a constant, then the sequence is called an equal sum sequence, and this constant is known as the common sum of the sequence. Given that the sequence $\{a_n\}$ is an equal sum sequence, $S_n$ is the sum of the first $n$ terms, and $a_1=2$, the common sum is $5$, find $S_9$.

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