
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
For an ellipse $\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1$ ($a > b > 0$), the distances from any point on the ellipse to the two foci are $d_1$ and $d_2$, respectively, and the focal distance is $2c$. If $d_1$, $2c$, and $d_2$ form an arithmetic sequence, then the eccentricity of the ellipse is (　　).

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