
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 the ellipse $\frac {x^{2}}{a^{2}}+ \frac {y^{2}}{b^{2}}=1$ ($a>b>0$) has an eccentricity of $\frac { \sqrt {2}}{2}$, and one endpoint of the minor axis is M(0,1). The line $l$ passing through the left vertex A of the ellipse intersects the ellipse at another point B.  
(I) If $l$ intersects the line $x=a$ at point P, find the value of $\overrightarrow {OB} \cdot \overrightarrow {OP}$.  
(II) If $|AB|= \frac {4}{3}$, find the slope angle of line $l$.

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