
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
A sequence is called a constant sum sequence if the sum of each term and its succeeding term is the same constant. This constant is referred to as the common sum of the sequence. Given that the sequence $\{a_n\}$ is a constant sum sequence with $a_1 = 2$ and a common sum of 5, find the value of the sum of the first 21 terms, $S_{21}$.

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