
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 $( \sqrt {x}- \dfrac {2}{x^{2}})^{n}(n\in\mathbb{N}^{*})$ where the sum of the binomial coefficients is $256$.  
$(1)$ Does this expansion contain a constant term? How many rational terms are there? And explain the reason.  
$(2)$ Find the term with the smallest coefficient in the expansion.

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