The formula to calculate the Eccentricity Index (EI) is:
\[ EI = \sqrt{1 - \left( \frac{b^2}{a^2} \right)} \]
Where:
Let's say the length of the semi-major axis (a) is 10 and the length of the semi-minor axis (b) is 6. Using the formula:
\[ EI = \sqrt{1 - \left( \frac{6^2}{10^2} \right)} \]
We get:
\[ EI \approx 0.80 \]
So, the eccentricity index is approximately 0.80.