Start with a circle and draw two tangent circles inside. The (black) inner tangent lines to the smaller circles intersect the large circle. The (red) lines through these intersection points are parallel to the (green) outer tangents to the small circles.
A long time ago I worked on this theorem, but I never knew the name. Without a name it's difficult to find more information. Does anyone know if this theorem has a name and where I can find more information about it?
Best Answer
Even more is true for this theorem. Check out this drawing from Arseniy Akopyan wonderful book of Geometry in Figures (Second, extended edition, 2017). On page 65 we find Figure 4.7.29)
In the foreword, Arseniy Akopyan writes
He nevertheless provides source for many of the figures in the end of his book. Unfortunately for Figure 4.7.29 he doesn't provide such a reference.
This leads me to the answer: Probably it doesn't have a name (like many "geometry theorems").