# Eccentricity

## Eccentricity of an Ellipse

The eccentricity of an ellipse is the number that expresses its degree of roundness.

**c** is half of the distance between the foci.

**a** is half of the length of the major axis.

## Eccentricity of a Hyperbola

Eccentricity measures the degree of the opening of the branches of the hyperbola.

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