
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 person had 21 trees planted in the shape of a triangle. If this person wants to fence off a triangular area of their land with trees at the corners, how many ways can they do this? 

Dashed lines show three possible ways. How many ways are there in total?

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