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# Semi-major and semi-minor axes

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In geometry, the **semi-major axis** is the distance from the center of an ellipse to the farthest point on the perimeter of the ellipse. The semi-major axis is half of the major axis, which goes all the way across the ellipse at the widest part. The **semi-minor axis** is a line segment that is at 90 degrees with the semi-major axis. For the circle, the lengths of the semi-axes are both equal to the radius.