
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 $f(\alpha)= \frac {\sin (\pi-\alpha)}{\sin ( \frac {\pi}{2}+\alpha)}$.  
(I) Find the value of $f( \frac {4\pi}{3})$;  
(II) If angle $A$ is an interior angle of $\triangle ABC$ and $f(A)= \frac {3}{4}$, find the value of $\cos ^{2}A-\sin ^{2}A$.

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