
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 plane rectangular coordinate system xOy, the vertex of angle α coincides with the coordinate origin, and its initial side coincides with the non-negative semi-axis of the x-axis. Its terminal side passes through point P(-3/5, -4/5).
(1) Find the value of $sin(α+ \frac {π}{3})$;
(2) If angle β satisfies $sin(α+β)= \frac {5}{13}$, find the value of cosβ.

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