
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 that $α$ is an angle in the second quadrant and $\sin α= \frac {4}{5}$:
$(1)$ Find the value of $\tan α$;
$(2)$ Find the value of $\dfrac {\sin (π+α)-2\cos ( \frac {π}{2}+α)}{-\sin (-α)+\cos (π-α)}$.

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