
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 the function $f(x)= \frac {1+a\cdot 2^{x}}{2^{x}+b}$ is an odd function, and the graph of function $f(x)$ passes through the point $(1,3)$.  
(I) Find the values of the real numbers $a$ and $b$;  
(II) Find the range of the function $f(x)$ when $x < 0$.

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