
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
In the Cartesian coordinate plane, $O$ is the origin, and two points $A(2,1), B(-1,-2)$ are given. If point $C$ satisfies $\overrightarrow{OC}=s\overrightarrow{OA}+t\overrightarrow{OB}$, and $s+t=1$, then the trajectory equation of point $C$ is ________.

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