
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 $\triangle ABC$, the sides opposite to angles $A$, $B$, $C$ are $a$, $b$, $c$ respectively, and $C= \dfrac {2π}{3}$, with $a^{2}-(b-c)^{2}=(2- \sqrt {3})bc$.
(I) Find the measure of angle $B$;
(II) If the arithmetic sequence $\{a_{n}\}$ has a non-zero common difference, and $a_{1} \cdot \cos 2B=1$, and $a_{2}$, $a_{4}$, $a_{8}$ form a geometric sequence, find the sum of the first $n$ terms, $S_{n}$, of the sequence $\{\dfrac {4}{a_{n}a_{n+1}}\}$.

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