
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 $\triangle ABC$, $\angle A=90°$, $AB=1$, $AC=2$. Let points $P$ and $Q$ satisfy $\overrightarrow {AP}=\lambda \overrightarrow {AB}$, $\overrightarrow {AQ}=(1-\lambda) \overrightarrow {AC}$, where $\lambda\in\mathbb{R}$. If $\overrightarrow {BQ}\cdot \overrightarrow {CP}=-2$, then $\lambda=$ ______.

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