[1] For D on the tangent at A

[2] Triangles DA

[3] DA

[4] The circle through A

[5] Read inversely: Every circle tangent to the ellipse at A

To prove [2] notice that (angle(B

Notice that circle k, passing through {A

This remark gives an easy procedure to construct the oscillating circle of a conic at a point A

[1] Draw a main chord A

[2] Reflect the tangent A

[3] The center of the osculating circle (o) is the intersection point of the normal at A

Notice that, in case there are two main chords through A

InvolutiveHomography.html

ReflexionOnConics.html

RotationOnConics.html

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