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Finding the foci of an ellipse

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See also the animated version.

After doing this Your work should look like this
Start with a given ellipse with its major and minor axes shown. Geometry construction with compass and straightedge or ruler or ruler
1.  With the compasses' point on the center, set the compasses' width to half the width (major axis) of the ellipse. Geometry construction with compass and straightedge or ruler or ruler
2.  Move the compasses' point to one end of the minor axis of the ellipse and draw two arcs across the major axis. Geometry construction with compass and straightedge or ruler or ruler
3.  Where these arcs cross the major axis are the foci of the ellipse. Label them F1, F2. Geometry construction with compass and straightedge or ruler or ruler
4.  Done. The two points F1, F2, define the foci of the ellipse.  
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Other constructions pages on this site

Lines

Angles

Triangles

Right triangles

Triangle Centers

Circles, Arcs and Ellipses

Polygons

Non-Euclidean constructions