
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 $f(\alpha)= \frac {\cos (\pi-\alpha)\cos ( \frac {3\pi}{2}+\alpha)}{\sin (\alpha-\pi)}$  
$(1)$ Simplify $f(\alpha)$;  
$(2)$ If $\alpha$ is an angle in the second quadrant, and $\cos (\alpha- \frac {\pi}{2})= \frac {3}{5}$, find the value of $f(\alpha)$.

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