
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 $f(x)=\cos x+\cos (x+ \frac {π}{2}).$
(1) Find $f( \frac {π}{12})$;
(2) Suppose $α$ and $β∈(- \frac {π}{2},0)$, $f(α+ \frac {3π}{4})=- \frac {3 \sqrt {2}}{5}$, $f( \frac {π}{4}-β)=- \frac {5 \sqrt {2}}{13}$, find $\cos (α+β)$.

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