
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 "constant sum sequence": In a sequence, if the sum of each term and its next term is the same constant, then this sequence is called a constant sum sequence, and this constant is called the common sum of the sequence. It is known that the sequence $\{a_n\}$ is a constant sum sequence, and $a_1 = 2$, the common sum is 5. The value of $a_{18}$ is \_\_\_\_\_\_\_\_, and the sum of the first 21 terms of this sequence, $S_{21}$, is \_\_\_\_\_\_\_\_.

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