
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
Given that $a\_n = \frac{n(n+1)}{2}$, remove all numbers in the sequence $\{a\_n\}$ that can be divided by $2$, and sort the remaining numbers in ascending order to form a new sequence $\{b\_n\}$. Find the value of $b\_{21}$.

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