For C moving on the ellipse, the geometric locus of point B is the red line (omothetic with ratio 2 of the [normal pedal] of A with respect to the ellipse). The envelope of lines BC is called [caustic] of the ellipse, with respect to the point A.

For a picture of the pedals (tangential and normal) of the ellipse, look at the file: Pedals.html .

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