
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 an arithmetic sequence $\{a_n\}$ where all terms are distinct, the sum of the first $7$ terms is $70$, and $a_3$ is the geometric mean of $a_1$ and $a_7$.  
(I) Find the general formula for the sequence $\{a_n\}$.  
(II) If the sequence $\{b_n\}$ satisfies $b_{n+1}-b_n=a_n$ for $n\in\mathbb{N}^*$ and $b_1=2$, find the sum of the first $n$ terms of the sequence $\left\{ \frac{1}{b_n} \right\}$, denoted as $T_n$.

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