
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 oblique triangular prism $A_{1}B_{1}C_{1}-ABC$, where $BC$ is the midpoint of $M$, $\overrightarrow{{A_1}{B_1}}=\overrightarrow{a}$, $\overrightarrow{{A_1}{C_1}}=\overrightarrow{b}$, $\overrightarrow{{A_1}A}=\overrightarrow{c}$, then $\overrightarrow{{B_1}M}$ can be expressed as ______ in terms of $\overrightarrow{a}$, $\overrightarrow{b}$, and $\overrightarrow{c}$.

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