
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
A student simplified $\left(a+2b\right)^{2}-\left(a+b\right)\left(a-b\right)$ as follows:  
Solution: Original expression $=a^{2}+4b^{2}-(a^{2}-b^{2})$ (Step 1)  
$=a^{2}+4b^{2}-a^{2}-b^{2}$ (Step 2)  
$=3b^{2}$ (Step 3)  
(1) The student made a mistake starting from step ______.  
(2) Please provide the correct solution to this question and find the value of the algebraic expression when $a=-\frac{1}{2}$ and $b=2$.

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